Try harder to collect and emit errors on validation failures.
Try harder to collect and emit errors on validation failures.

There's no need to wait for a NOTIFY, as `validate_sync()` runs synchronously.
So just check the return value for errors, and use excpetion handling to catch
other kinds of errors. This allows us to catch and emit all errors and also
allow execution to finish validating all syncs (should it be validating
multiples). Closes #65.

ezekiel/bucardo,ezekiel/bucardo,ezekiel/bucardo