Show something better than "Bundle::GemfileNotFound" when bundle is run in the wrong directory.

Currently, users who just run 'bundle' in the wrong directory will just see "Bundle::GemfileNotFound" instead of a friendly error.  This pull request fixes that by adding a message to the GemfileNotFound exception raised from Bundler::Settings.  It also adds a spec for Bundler::Settings that tests that the exception with the message is raised.