cmd/go: add prefix to errors when using -x

[QuLogic]

For normal compiles, the output is sparse, and so errors are quite obvious. The fact that sometimes compiling will continue on to other files is not too important.

However, in verbose mode, there is a lot of output and it is very very very difficult to find an error in the sea of verbosity, especially when multiple files were compiled after the actual error. If errors had an error: prefix or similar, it would be far easier to find the source of the problem.

[mvdan]

What errors? Build errors? And what verbose mode do you mean?

[QuLogic]

Yes, build errors. Sorry, not verbose mode, but with the -x flag of go build. Now that I see it's not directly a verbose flag, per se, maybe this isn't so obvious a thing to link to it.

[seankhliao]

go build -x ./... 2>&1 | grep '^[^\s]*\.go' works today (errors start with a path to a go file.

[QuLogic]

Ah, that is useful and works locally at least. But it's annoying if you're viewing a build long on some CI or build server in a browser, since you can't use regex searches.