Change forked run method to consider some errors as success

Some errors caused by a mutation, like ArgumentError and NameError indicate that the mutation results in code that fails with the specs. I think these should be considered successful.

@mbj I opened this PR for discussion. I mentioned it in another comment, but I have tested this with axiom and it resolved a few of the syntax error problems that were reported. The problem is I'm not sure if this has any negative side effects, so I wanted to run this by you since you're much more familiar with the whole system.