More extensive debugging for plugin build failure

[rmulhol]

Right now when the plugin fails to build we get fairly obscure error messages like:

{
  "SubCommand": "compose",
  "file": "/go/src/github.com/vulcanize/vulcanizedb/cmd/compose.go:124",
  "func": "github.com/vulcanize/vulcanizedb/cmd.compose",
  "level": "fatal",
  "msg": "unable to build .so file: exit status 1",
  "time": "2019-08-10T10:50:30Z"
}

It would be nice if we had more detailed information about why the build failed.

[i-norden]

That error is coming from the exec.Command() call here, I'm not sure how much we can modify it without digging into and modifying core packages.

Should note that the need for the hacky transient build environment for the plugins was due to this issue golang/go#20481 and now that we are using go-modules we don't have vendored dependencies.

[rmulhol]

@i-norden agreed, this is a tough nut to crack - but hopefully easier with go modules.

One thing I've been exploring is seeing if we can do the go build --buildmode=plugin step from the Makefile, but I'm still wrapping my head around how to build the transformerExporter.go file in that scenario. Seems a little weird to ./vulcanizedb compose & make build_plugin & ./vulcanizedb execute but I haven't figured out whether there's a simpler path 🤔