remove try/except from add_subscription

[FlorianLudwig]

DISCLAIMER: I do not understand why this was there in the first place.

I am sure there is (or was) a good reason.

The issue is, the "try" block is way to big and covers a lot code with a rather broad exception - catching exceptions that should not. This is part of the problem why we didn't notice #60 earlier, as we would have got:

  File "amqtt/plugins/topic_checking.py", line 73, in topic_filtering
    allowed_topics = self.topic_config["acl"].get(username, None)
KeyError: 'acl'

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As the diff is a bit hard to read on github: I just removed the try/except

[HerrMuellerluedenscheid]

Oh yeah that looks bad. Good to remove it and if problems occurr to catch them more selectively. It's still marked draft but I think this is reasonable the way it is. Also none of the tests fail.

[FlorianLudwig]

None of the tests fail,... but the test coverage on that part is a bit sketchy as we didn't catch the error that the most basic version of our mqtt server broke 😓

See also #43 (comment) ;P

[not-f-elsner]

LGTM