Improve handle_errors context manager
#657
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1. doing the abort() inside of `handle_errors` is useful, because currently, every call of this context manager resides in a `try: … except PycroftError: return default_response()`-block. 2. The `session.begin_nested()` seems convenient, but just hides something important from the person programming the view. Since in the view, `session.commit()` has to be called explicitly as well, it is only consistent to also call `session.begin_nested()` explicitly. Even more importantly, blindly wrapping the whole cm body in `begin_nested` has two disadvantages: a) It discourages „commit often“, which is favorable to long-running transactions b) It allows you to think `commit` inside this context will just work, while this actually will cause problems in production (undetected in tests, because there `commit()` is a NOP): sqlalchemy disallows committing while in a subtransaction. Thus, it makes more sense to let the user call `begin_nested` explicitly.
This will be removed after fixing all the usages
This will not cause errors to be unhandled, because exiting out of the `begin_nested()` context manager will cause a `flush()`, triggering all of the possible errors that might occur in the transaction.
It has been replaced by the `handle_errors` context manager.
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