Fix for cf.aggregate failures when field units are non-valid

[davidhassell]

Fixes: #229

[sadielbartholomew]

All good. As indicated for the desired solution in the corresponding issue:

cf.aggregate needs to return, unaggregated, the fields with bad units, and report that it has done so

which has been achieved here, as tested by applying override_units("mangoes", inplace=True) on a field and observing that it reports Unaggregatable 'eastward_wind' fields have been output: Non-valid units <Units: mangoes> upon unsuccessful (without erroring) attempt to aggregate it with itself.

I have made one optional minor suggestion made not pertaining to the functionality, as below.

[sadielbartholomew]

Actually, ignore the conclusion from my previous review comment, sorry - I meant to say we could do with a test for the bad units case to check for the right result and log message! I suppose that could be added in a later PR, though.