Don't swallow validator stacktraces.

This won't affect the Validator app output, just when it's run directly
from main().
Fix a couple of bugs in the SimpleSentenceForm. Add a regression test.
Merge branch 'master' of https://github.com/ggp-org/ggp-base into simple-sentence-form-fixes
Merge pull request #33 from ggp-org/simple-sentence-form-fixes

Simple sentence form fixes