SCUBA2: use SkyRef when fudging pointing position

The _SHIFT_IMAGES_ primitive only used the SkyRef attribute if
the source was a recognised calibrator, which meant that
_FUDGE_CALIBRATOR_POSITION_ would not get offsets for non-calibrator
pointing observations.  Therefore a new argument USE_SKYREF
has been added to _SHIFT_IMAGES_ to instruct it always to use
the SkyRef position.  This argument is passed from
_FUDGE_CALIBRATOR_POSITION_ through _MAKEMAP_FRAME_ via a
new argument SHIFT_USE_SKYREF of that primitive.
SCUBA2: make failure to fudge pointings fatal

_FUDGE_CALIBRATOR_POSITION_ now calls orac_termerr if it is
processing a pointing observation and is not able to determine
pointing offsets.  This is because pointing observations would
tend to be carried out when the pointing needs to be corrected,
and we do not want to include poorly pointed pointings in JSA
HEALPix tiles because they may smear the image.  Therefore
the recipe should fail when trying to reduce a pointing and the
position cannot be fudged.  The alternative would be to mark
these observations somehow for subsequent exclusion from
co-adds.

The addition of this check was agreed by JTD.