In the short term, just catch the SystemExit.

In the slightly longer term, we could certainly provide a subclass, say, ErrorRaisingArgumentParser, that overrides .exit and .error to do nothing but raise an exception with the message they would have printed. We'd probably have to introduce a new Exception subclass though, maybe ArgumentParserExit or something like that.

Anyway if you're interested in this, please file a new ticket (preferably  with a patch). Regardless of whether we ever provide the subclass, we certainly need to patch the documentation to tell people how to override error and exit.