Return error message(s?) via JSON for vim-go

[stapelberg]

Split out of #1

From @fatih:

func foo() error {
	bar()

	// no return yet as I'm still developting this
}

func bar() error {
	return errors.New("foo")
}

When I put my cursor on top of bar() and call :GoExpandErr it returns an error saying:

2017/12/28 17:58:43 ignoring type-checking error: demo.go:54:1: missing return
{"start":51,"end":51,"lines":["\tif err := bar(); err != nil {","\t\treturn err","\t}"]}

It actually returns something. However this is wrong. The error should be part of json and we should be able to decide whether we can change the buffer or not. I think the error message above can be mitigated, but not sure how expanderr implemented it.