Don't swallow LoadErrors when requiring a dashed gem

This looks like #1807 

when raising a LoadError in a required gem, bundler will rescue it to try and see if it came from his own namespaced require or from the required file.

If it comes from the dashed require it will re-raise the original exception in case the gem was dashed but the first require was ok but raised a LoadError after.

However, the exception is swallowed if we require a dashed gem without the proper file name, for example gem name : 'foo-bar' and file architecture : 'foo/bar.rb'

This PR raise the exception back in case it really comes from inside the namespaced file.