add option to bomb out when null model is detected

[o-smirnov]

Model columns can't be null. They're only null when (a) casacore/python-casacore#130 has its teeth in you, or (b) WSClean/DDFacet did not fill in the model properly (possibly because you had too many flags on the MS when predicting).

Both cases are Very Bad News actually, so I think the red warning should be upgraded to an exception.

(Or maybe add a parset option to throw an exception or not? Default being to throw.)

[cyriltasse]

or (c) when the sky itself is empty... :))

[o-smirnov]

or (c) when the sky itself is empty... :))

Which is also an excellent reason to stop calibrating!