Refactor progress/error handling

[fredemmott]

• if we're dealing with a list of tests, return a container of result-or-exceptions
• if not, throw
• add clear hierarchy of progress events, events with files, events with classes, events with tests, etc.
• report parse errors as test failures
• if there's in appropriate number of test classes in a file:
  • report it as a failure
  • add number of test class
  • list all the classes and why they are/aren't test classes
• separate verbose/concise error reporting to subclasses
• remove separation of result printing and progress printing
• generally uses is to distinguish between different kinds of events; also offers enums where completeness checks are valuable

The final summary printing should probably also move to the OutputHandler, but given this diff is already big, I left it out for now.

This:

• provides a better base for:
  • unit testing the unit test framework :p
  • adding more output formats, such as JSON
• improves some common usability problems (see https://gist.github.com/fredemmott/32390c84c85c185a3519c39c27d05baf)

refs #88

[fredemmott]

Test failures on 4.8; will investigate tomorrow. If it's trivial, will fix, otherwise will bump minimum version