Show found generated file on comparison failure.

Currently a failure to generate the expected output on the part of an annotation processor or other in-javac generator is an exception, but the code that was generated is not shown at any point in the output.  This change shows the generated code _if_ code was generated under the expected path.

This change does not attempt to scan generated files for edit distance - it does a very naive check of whether code was generated at all at the expected output path, and shows it in the error.