Adds more testing/error verbosity, fixes nested previous records bug

[echeng06]

Mostly just testing for weird situations, but also makes two algorithmic updates:

• Fixes issue where datastore was not being set for the temp validator in _check_subschema_valid, causing nested conditions with previous records to not evaluate correctly (e.g. A1 EDUC case, where compare_with previous record was nested inside a temporalrules)
  • Fix was to just assume any sub-validator will use the same datastore/primary_key and set it
• Updates compatibility errors to be more verbose - before we were not reporting what the then/else conditions were and so it was impossible to tell what was wrong if a then/else condition failed. Unfortunately that does mean the error messages are longer/uglier though.
  • Before there were also two COMPATIBILITY_FALSE and COMPATIBILITY_TRUE error messages that appeared to be the exact same thing, so I changed one of them to be COMPATIBILITY_ELSE to report when an else condition failed

[hewanadun]

Looks good!