run: capture error messages from panics

[bhcleek]

Capture error messages from panic. I'm not 100% sure that this is the right course of action. It might be better to treat panics as successes, because non-zero exit status from go run are generally discarded except for compiler errors. This change is inconsistent with that fact and may lead people to expect that non-zero exit statuses from the command being run should always populate the quickfix list.

Fixes #2328