WIP: Improved camera error handling by adding new camera & focuser property to Pyro interface

[AnthonyHorton]

Add new camera and focuser properties to the Pyro interface so that they can be used in huntsman-pocs.

Specifically:

1. Camera .can_take_internal_darks property (read only). This isn't actually anything to do with error handling but it's in POCS develop now so should be accessible for huntsman-pocs remote cameras too.
2. Camera .exposure_error property (read only). This will be None unless the most recent exposure taken by the camera raised an exception, in which case it will be equal to the string representation of that exception. By checking this after an exposure (or observation) has completed it is possible for calling code to detect cameras errors and respond accordingly, e.g. deleting failed cameras from the camera list.
3. Focuser .autofocus_make_plots property (read/write), can be used to check or set whether to produce the autofocus plots. This can still be overriden by setting the make_plots argument of the call to autofocus().
4. Focuser .autofocus_error property (read only). This will be None unless the most recent autofocus taken by the focuser raise an exception, in which case it will be equal to the string representation of that exception. By checking this after an autofocus sequence has complete it is possible for calling code to detect camera errors and respond accordingly, e.g. deleting failed cameras from the camera list.

Note: Apart from .can_take_internal_darks these new properties do not exist in POCS develop, only in the https://github.com/AnthonyHorton/POCS/tree/error-handling branch. See PR panoptes/POCS#1003 for more details.

[AnthonyHorton]

These changes should at least partially fix #205.

[danjampro]

@AnthonyHorton Is this still under development?