Modify error paths to be relative to the working directory

[dradtke]

This commit updates gb to automatically modify error paths in a command's output to be relative to the user's working directory, enabling the use of tools like Vim's quickfix to help navigate errors. Before, it would output directory information on individual lines, and the line containing the error would mean nothing to an editor since it would only be the file's base name, but with this patch, gb error output is now easily and correctly parsed by editors.

This is a very simple change tailored to my specific use case, and there are probably some changes that could be made to have it integrate better with the rest of gb, but it works, and is very straightforward in its functionality.

[davecheney]

Thank you for this proposal. However, I would like to see the solution implemented as per #406