PICARD: warn or quite for small numbers of bolometers in map

Maps made with very few bolometers are usually not good enough to
detect the source and may have other issues. Issue a warning if the
number of bolomters is between 50 and 100 (kind of arbitrary I
suppose), but terminate with an error if less than 50.
SCUBA2: trap null results and issue warning/error

When aperture photometry fails, the sum can still be defined but with
meancount and meanerr both being 0, the subsequent calculations will
fail. So protect against divide-by-zero and issue an error if
meancount is 0.