Roll back transaction on failed ExecContext

[egoodhall]

Adds a call to sqlTx.Rollback() when an error is returned from compat.ExecContext in the SQLite adapter. This addresses #669 SQLite Adapter doesn't roll back transaction on error, which I ran into while writing a SQLite-backed web app.

This seems to have also addressed https://github.com/upper/db/issues/636 - I was able to remove the case that skipped the test for SQLite.