Remove common error handler from ExceptionMiddleware

[julienschuermans]

This PR removes the catch-all error handler for unhandled exceptions from the ExceptionMiddleware.

Since the introduction of @nielsbox' ServerErrorMiddleware, the duplication of this feature has become kind of confusing / unnecessary.

[RobbeSneyders]

Thanks @julienschuermans.

Agree with the goal of the PR, but seems like we need some more work to fix this since the tests are failing.

As far as I understand the issue, we are wrapping the error_response in the ServerErrorMiddleware with our connexion_wrapper from the ExceptionMiddleware which returns a coroutine, while the ServerErrorMiddleware expects a normal function here. We could solve this by just implementing the relevant part (translating between Connexion and Starlette) synchronously in the error_response method.

Another difference is that the ServerErrorMiddleware still raises the error after returning the error response. So we'll need to update our tests to accommodate this and catch the error. We also need to check what the effect of this is when running the application with different servers.