Fix several minor issues:
1) Internal_error_handler::handle_condition() had an unused argument
2) Several error handlers did not take advantage of the new possibility
   to downgrade errors to warnings - they instead did this manually.
3) The handler error handler for DROP TABLE by mistake used the
   handler error number rather than the mysql error number when downgrading
   from errors to warnings.
4) Code consistency is improved
5) THD::raise_condition() had a disabled assert that is reenabled
   (it passes on current trunk with default MTR tests)