_VerifyErrors()._Error() and _ErrorIf() are now consistent

_Error() didn't contain the logic for demoting errors to warnings and for
marking an operation as failed. Now _ErrorIf() is just a minimal wrapper
for _Error().

Unit tests included.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>