Properly display errors that are not 'Errors'

This is was initially brought up to fix a problem with YAML errors
being reported as 'undefined'. It should now show the result of the
object's toString, or the object stringified if the toString looks
unhelpful.
added terminal_printer 'normalize_error' method
Merge pull request #370 from nathanharper/master

Properly display errors that are not 'Errors'