Raise a meaningful exception for missing gemspecs

For the path option in a gem dependencies file, if the gemspec was
missing a NoMethodError was raised which didn't make any sense.

Now a Gem::GemNotFoundException is raised which has a message that will
help the user understand what they need to fix.
Add pretty_print for Gem::List
Support minitest 4 output in autotest

Without this minitest does not operate correctly as it cannot discover
the method and class names.