SCUBA2: check amplitude first, issue error if negative

A negative amplitude is serious so issue an error rather than just a warning
SCUBA2: do not mark Frame as bad if FCF calculation fails

If the FCF calculation has failed then the S/N is probably just too
low, especially at 450 um: that's not the same thing as bad
data. Frames should only be marked bad if the image has blank pixels
near the centre or if the variance is bad etc.
SCUBA2: add beam assessment, print message in coloured text

Compare the fitted beam size with the main beam component of the
telescope, and issue warnings if the fitted beam exceeds the telescope
beam by 10%. Colour the reported beam size accordingly: green if <10%
larger, yellow if between 10 and 20%, red if > 20%.
SCUBA2: check magnitude of offsets and compare az/el values

Warnings are issued if the the magnitude of the pointing offset
exceeds 10 arcsec and/or the derived azimuths and elevations differ by
5 arcsec.

Colour-code the info message for users: green if the magnitude is
<10", yellow if between 10 and 20", red if >20".