Make the error messages that result from referencing
nonexistent traits consistent, and add a test.
Address @catamorphism's error message grammar nit.
auto merge of #7572 : Dretch/rust/missing-trait-message-followup, r=pcwalton

This a followup to #7510. @catamorphism requested a test - so I have created one, but in doing so I noticed some inconsistency in the error messages resulting from referencing nonexistent traits, so I changed the messages to be more consistent.