Bug 1043524 - Define group variable is an existing parser is passed; r=jgraham

While I was here, I removed the try..except in favor of failing fast.
Before, we were only making cosmetic changes. Now, we actually need the
group for mach to work. It makes sense to stop ignoring errors.

This fixes a regression from d53d1c6cdf13 / bug 951733.

--HG--
extra : rebase_source : aeb6c99a945eaff32cf86fa481d03c0c8dee593a
extra : amend_source : f1e29385bf8ca8ac250a04f81e7e955c4152b476