Crash when vault secret is not found

[hairyhenderson]

This is a regression in v2.0.0.

When attempting to reference a nonexistent secret, gomplate panics now instead of giving an error.

Old behaviour (v1.10.0)

$ gomplate-1.10 -d vault=vault:///secret/path/to/secret -i '{{ ds "vault" "mysecret" }}'
2017/08/31 15:50:34 Couldn't read datasource 'vault': Unexpected HTTP status 404 on Read from /secret/path/to/secret/mysecret: {"errors":[]}

New (buggy) behaviour (v2.0.0)

$ gomplate -d vault=vault:///secret/path/to/secret -i '{{ ds "vault" "mysecret" }}'
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x17297d1]

goroutine 1 [running]:
text/template.errRecover(0xc420159aa0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:140 +0x1ec
panic(0x187e0a0, 0x1ebd620)
	/usr/local/Cellar/go/1.8.3/libexec/src/runtime/panic.go:489 +0x2cf
github.com/hairyhenderson/gomplate/vault.(*Vault).Read(0xc420110010, 0xc4204a4f00, 0x3e, 0x19a7eb6, 0x1, 0x7fff5fbff589, 0x19, 0xc4204a4f00)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/vault/vault.go:53 +0xe1
github.com/hairyhenderson/gomplate/data.readVault(0xc4202c9420, 0xc4202c6aa0, 0x1, 0x1, 0xc420197eb0, 0xc4202c1f01, 0x1e, 0xc420158e18, 0x105a174)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data/datasource.go:368 +0x399
github.com/hairyhenderson/gomplate/data.(*Data).ReadSource(0xc4202c6800, 0xc4202c9420, 0xc4202c6aa0, 0x1, 0x1, 0x1, 0x320194b40, 0x1, 0x28, 0x38)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data/datasource.go:256 +0x19a
github.com/hairyhenderson/gomplate/data.(*Data).Datasource(0xc4202c6800, 0x7fff5fbff581, 0x5, 0xc4202c6aa0, 0x1, 0x1, 0xc420158ff8, 0x100fe8c)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data/datasource.go:205 +0xc3
github.com/hairyhenderson/gomplate/data.(*Data).Datasource-fm(0x7fff5fbff581, 0x5, 0xc4202c6aa0, 0x1, 0x1, 0x0, 0x0)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/funcs/data.go:22 +0x5c
reflect.Value.call(0x1852080, 0xc4202c6830, 0x13, 0x19a886c, 0x4, 0xc4202bfc20, 0x2, 0x2, 0xc420159890, 0x10e72b2, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/reflect/value.go:434 +0x91f
reflect.Value.Call(0x1852080, 0xc4202c6830, 0x13, 0xc4202bfc20, 0x2, 0x2, 0x1e8d900, 0xc420264e40, 0x17fc760)
	/usr/local/Cellar/go/1.8.3/libexec/src/reflect/value.go:302 +0xa4
text/template.(*state).evalCall(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0x1852080, 0xc4202c6830, 0x13, 0x1e8d540, 0xc4202bfb60, 0x7fff5fbff57d, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:668 +0x51f
text/template.(*state).evalFunction(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0xc4202bfb90, 0x1e8d540, 0xc4202bfb60, 0xc420264e80, 0x3, 0x4, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:536 +0x18e
text/template.(*state).evalCommand(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0xc4202bfb60, 0x0, 0x0, 0x0, 0xc42001e600, 0x7fff5fbff57a, ...)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:433 +0x6cd
text/template.(*state).evalPipeline(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0xc42027e230, 0x100eb02, 0xc4202bfbf0, 0x30)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:406 +0xf2
text/template.(*state).walk(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0x1e8d3c0, 0xc4202bfbc0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:232 +0x55a
text/template.(*state).walk(0xc420159a20, 0x185dcc0, 0x1eebae8, 0x16, 0x1e8d720, 0xc4202bfb30)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:240 +0x139
text/template.(*Template).execute(0xc420264d80, 0x1e85d80, 0xc42000c018, 0x185dcc0, 0x1eebae8, 0x0, 0x0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:195 +0x20a
text/template.(*Template).Execute(0xc420264d80, 0x1e85d80, 0xc42000c018, 0x185dcc0, 0x1eebae8, 0x0, 0xc4201142d0)
	/usr/local/Cellar/go/1.8.3/libexec/src/text/template/exec.go:178 +0x53
main.(*Gomplate).RunTemplate(0xc4202bfa40, 0x7fff5fbff57a, 0x2c, 0x1e85d80, 0xc42000c018)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/gomplate.go:30 +0x23e
main.renderTemplate(0xc4202bfa40, 0x7fff5fbff57a, 0x2c, 0x19a7eb4, 0x1, 0x0, 0x0)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/gomplate.go:66 +0xc1
main.processInputFiles(0x7fff5fbff57a, 0x2c, 0xc4202c6730, 0x1, 0x1, 0xc4202c6770, 0x1, 0x1, 0xc4202bfa40, 0xc4201f01e0, ...)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/process.go:23 +0x12f
main.runTemplate(0x1eccc00, 0x0, 0x0)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/gomplate.go:55 +0x22d
main.newGomplateCmd.func1(0xc4201ec6c0, 0xc420264d00, 0x0, 0x4, 0x0, 0x0)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/main.go:69 +0x7e
github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra.(*Command).execute(0xc4201ec6c0, 0xc42000e240, 0x4, 0x4, 0xc4201ec6c0, 0xc42000e240)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra/command.go:649 +0x457
github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc4201ec6c0, 0x2, 0x19a80e4, 0x2)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra/command.go:728 +0x339
github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra.(*Command).Execute(0xc4201ec6c0, 0xc420159f78, 0x0)
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/vendor/github.com/spf13/cobra/command.go:687 +0x2b
main.main()
	/Users/dee/gocode/src/github.com/hairyhenderson/gomplate/main.go:96 +0x46