errors are printed with full stacks

[BrianHicks]

We probably don't need to do that unless they're panics.

Example:

=== testing blackbox/test_graph_dependencies.sh ===
/var/folders/f6/l26tk3xs5pjczcd5k5pcrmk00000gn/T/converge.graph_dependencies.XXXXXXXXXX.LqjDA129 ~/code/go/src/github.com/asteris-llc/converge
WARN[0000] setting session-local token                   token=0e7c9a3a-6bc4-49fe-8e32-5147262db7e4
ERRO[0000] could not render                              component=rpc error=param is required
root/param.cn
github.com/asteris-llc/converge/render.Render.func1
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/render/render.go:69
github.com/asteris-llc/converge/graph.transform.func1
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/graph/graph.go:542
github.com/asteris-llc/converge/graph.rootFirstWalk
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/graph/graph.go:423
github.com/asteris-llc/converge/graph.transform
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/graph/graph.go:543
github.com/asteris-llc/converge/graph.(*Graph).RootFirstTransform
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/graph/graph.go:444
github.com/asteris-llc/converge/render.Render
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/render/render.go:75
github.com/asteris-llc/converge/rpc/pb.(*LoadRequest).Load
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/rpc/pb/loadrequest.go:41
github.com/asteris-llc/converge/rpc.(*grapher).Graph
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/rpc/grapher.go:36
github.com/asteris-llc/converge/rpc/pb._Grapher_Graph_Handler
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/rpc/pb/root.pb.go:647
github.com/asteris-llc/converge/vendor/google.golang.org/grpc.(*Server).processStreamingRPC
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/vendor/google.golang.org/grpc/server.go:686
github.com/asteris-llc/converge/vendor/google.golang.org/grpc.(*Server).handleStream
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/vendor/google.golang.org/grpc/server.go:770
github.com/asteris-llc/converge/vendor/google.golang.org/grpc.(*Server).serveStreams.func1.1
        /Users/brianhicks/code/go/src/github.com/asteris-llc/converge/vendor/google.golang.org/grpc/server.go:419
runtime.goexit
        /usr/local/Cellar/go/1.7/libexec/src/runtime/asm_amd64.s:2086 location=/Users/brianhicks/code/go/src/github.com/asteris-llc/converge/samples/testdata/cert.hcl runID=b331a9c7-2de8-4b4a-951d-9bd7453b5da0

This can probably be fixed with a custom formatter. (Printing errors that have passed through github.com/pkg/errors have %s and %v formatting options, %s just prints the error and %v has the stacktrace.)