meas.mosaic.updateExposure.applyMosaicResultsExposure will not if there is no mosaic solution

Furusawa-san pointed out that coaddDriver can produce outputs even if mosaicking has not been run though doApplyUberCal=True (which is the default value for HSC). This appears to be due to allowing null values of the wcs and ffp in updateExposure.py.

We should allow the functions in there to fail if the appropriate values are not available, or the user can be deceived as to what corrections have actually been applied.