cmd/vet: format of -shadow error message needs a patch

[stemar94]

Currently the output of vet -shadow gives something like the following:

shadowtest.go:17: declaration of local shadows declaration at shadowtest.go:16: 
shadowtest.go:24: declaration of global shadows declaration at shadowtest.go:8: 

Notice the colon space at the end of the line and the fact, that it doesn't get directly clear, which variable is meant.

I would like to propose a format as follows:

shadowtest.go:17: declaration of 'local' shadows declaration at shadowtest.go:16
shadowtest.go:24: declaration of 'global' shadows declaration at shadowtest.go:8

At least the colon space at the end shouldn't be protected by any format guarantee, since I guess it is a bug and isn't especially tested at the moment.
I think the highlighting of the variable name should also be done as long as the -shadow option is still experimental.

I have a CL ready.

[gopherbot]

CL https://golang.org/cl/20093 mentions this issue.