better output (or any output at all) during `bundle exec rspec` failure

[loganhasson]

If somebody runs learn on a lab that has rspec in the Gemfile, but hasn't bundled yet, it fails with no output. Let's get some output that mentions running bundle install.

[loganhasson]

actually, can't reproduce reliably...doesn't seem to happen all the time. many times it outputs bundle's standard error message.

[loganhasson]

actually does happen...if bundle isn't installed