`checkdata` management command should not call sys.exit directly

[olivierdalang]

Hey again (still me :-) will try to contribute some fixes if I can find these days)

checkdata's implementation calls sys.exit, which prevents calling it programmatically with django.core.management.call_command (at least without catching SystemExit exceptions, which could have side effects I guess ?).

As far as I understand, system exit is implemented by BaseCommand, so rather than calling sys.exit, we should either do nothing (on success) or raise CommandError (which accepts an exit code arg).

Cheers

[jerch]

Oh well again missing tests 😅 otherwise I'd have spotted that (e.g. updatedata has a few call_command test cases).