Allow `ImmediateBackend` to throw its errors

[RealOrangeOne]

If a task fails using the ImmediateBackend, this exception is currently swallowed and isn't accessible anywhere. This should be rectified by adding a logger.exception call to ensure the error is seen, and captured by error monitoring. This matches the behaviour of the database backend.

[RealOrangeOne]

Currently, Exception is caught. This may need extending to BaseException with cut-outs for KeyboardInterrupt

[jribbens]

Currently, Exception is caught. This may need extending to BaseException with cut-outs for KeyboardInterrupt

That would mean it catches GeneratorExit and SystemExit, why would you want that?

[RealOrangeOne]

If a task throws SystemExit for whatever reason, I don't want the process to terminate. It should be caught as if the task failed, and control continue.

KeyboardInterrupt only needs cutting out as a quality of life fix. There might be others like it, but I don't think SystemExit is one of them.

[jribbens]

Catching and squashing SystemExit is a bold strategy ;-) But if you really want to do that then explicitly catching Exception and SystemExit would seem the appropriate way.