Register Controller does not release mutex when exceptions are thrown within the yield

[josephnobes-stfc]

The register controller's context manager is added so that with can be used during field / register access to make mutex handling easier. However, the current code simply returns the mutex ownership after the yield:

    @contextmanager
    def acquire(self, blocking=True, timeout=-1):
        # Grab the lock with the supplied settings
        result = self._register_mutex.acquire(blocking=blocking, timeout=timeout)

        # Allow the caller to execute their 'with'. 'result' is needed so that
        # if the lock cannot be grabbed the user can handle it.
        yield result

        # Release the lock, if it was actually acquired
        if result:
            self._register_mutex.release()

This means that if there is an exception thrown within the yield, it skips the mutex release and the caller will permanently own the resource.

The yield should be surrounded with a try...except that will release the mutex before raising the exception so as not to mask it.

This was encountered in https://github.com/stfc-aeg/babyd-embedded/issues/7