Improve `__repr__` of AsyncStatus

[dperl-dls]

Callbacks to a bluesky plan often log the reason for the exit_status, such as for instance appending it to the ISPyB deposition. If the plan has failed because an exception occurred in any AsyncStatus, the only information that makes it through the bluesky FailedStatus and to the emitted document is <AsyncStatus errored>. With many possible AsyncStatuses this is not very enlightening. I propose adding information about the task to the __repr__, as well as information about the exception if it exists.

[dperl-dls]

Oops, sorry, wrong ophyd