Abins2D: improve user feedback if coverage settings are invalid

[ajjackson]

Is your feature request related to a problem? Please describe.
Abins2D allows parameters of 2D chopper spectrometers to be selected for MAPS, MARI or MERLIN; these are dispatched to the PyChop library packaged within Mantid.

If certain low-energy/high-speed settings are selected, the coverage area is zero and the data fills with NaN, breaking the broadening logic.

It would be helpful to raise a more informative error in such situations.

Describe the solution you'd like
At validation time, use PyChop to check that the instrument coverage parameters are permitted; if not, fail validation with a useful message.

Describe alternatives you've considered
Improve the error message at runtime by catching this scenario. This is still helpful, but may come after a slow calculation; early validation would be better.