SCUBA2: remove FWHMerr now that two-component fits will be unlikely

Two component fits are now a thing of the past (or at least a thing of
special interest to a small number of experimenters) so there is no
need to keep the entry for the secondary FWHM.

Also document Error_beam log entry in the POD.
SCUBA2: compare error fraction with empirical value

Compare the derived fraction with the "expected" value for a
perfectly-focussed telescope and report the percentage deviation from
that, colour-coded by the magnitude of the deviation.