[Idea/Draft/Proposal] Exception handling for server exceptions

Currently there are a lot of exceptions 'lost' in asyncio land because we schedule a lot of our coroutines as tornado callbacks, that's usually a pattern like `self.loop.add_callback(do_something_which_might_raise)` To not loose these exceptions, there is, for instance, the `log_errors` handler which logs the exception. Other than the logging system we currently do not have a method to retrieve these exceptions, neither for internal validation (e.g. tests) nor to propagate the exceptions programatically to users

I'm wondering if we can set up an [exception handler]( https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.set_exception_handler ) and forward all exceptions to the running server. These may then be forwarded to the client where we may raise an exception or issue a log message there.

Users might probably want to configure the behaviour for the different kinds of exceptions, e.g. ignore CommClosed but raise on KeyError, etc.