improve error handling in `bundle-open'

"bundle show" can emit "Could not find GEM-X.Y.Z in any of the sources"
but this wasn't being handled correctly.  Also properly distinguish
between the Gemfile not being found and a gem not being found.
improve error handling in `bundle-list-gems'

"bundle list" can emit "Could not find GEM-X.Y.Z in any of the sources"
or "Could not locate Gemfile" but these weren't being handled correctly.
Merge pull request #7 from aspiers/master

improve error handling in `bundle-open' and `bundle-list-gems'