Fix TypeError while unpickling TelegramError (and children)

[Delgan]

Hi.

This is a possible fix for #2105.

Let me know if you think this can be improved in any way.

[Bibo-Joshi]

Hi. Thanks for the quick PR! I left two comments for you.

[tsnoam]

@Bibo-Joshi If we ever introduce a new exception which inherits from TelegramError, would it be covered by the unitests here?

Maybe we also need a unitests which looks at telegram.error.TelegramError.__subclasses__() and make sure that all subclasses are known and properly covered?

[Bibo-Joshi]

@Bibo-Joshi If we ever introduce a new exception which inherits from TelegramError, would it be covered by the unitests here?

Maybe we also need a unitests which looks at telegram.error.TelegramError.__subclasses__() and make sure that all subclasses are known and properly covered?

It wouldn't. But we might not be directly inheriting from TGError, so we would need a recursive loop. Should be doable, just not with parametrize. I can have a look, when I'm back at a keyboard and had some sleep :D

[tsnoam]

@Bibo-Joshi A basic and very naive unitest, yet serving the purpose would use the subclasses method in order to retrieve a list of methods and compare it to a pre-known const.
If the lists are not equal it means that we've added a new Exception and thus we need to make sure it has a unitest of its own (of course a comment all these would require a nice comment so people would know why we do this "stupid" things in the test)

Basically the idea is to make us remember that we need to implement un/pickling...

[Bibo-Joshi]

Added the "meta-test" and in the process found that there is a TelegramDecryptionError in passport/credentials.py :D That one in fact is a bit tricky as it passes TelegramDecryptionError: {message} to super().__init__. But I didn't want to fiddle with that behaviour, so I just added a self._msg that saves the actual message on init and is returned on __reduce__.

[Bibo-Joshi]

Thank your for your contribution @Delgan :)