Handle crashes in generation functions

[Fleex255]

Currently Answerable explodes if a @Generator function crashes. This is important to handle because student constructors called by receiver generators could have crash bugs or even desirably throw exceptions in some circumstances.

testGenerationFailed added in 87db240.

[JKTKops]

This should be attacked once we've settled on a structure for the testing loop. At this point we seem to be leaning towards an iterator-based approach, so questions of how an iterator reacts to a generation failure need to be answered.