SCUBA2: only throw an error if the Frame is good

If the Frame is bad then there is no need to throw an error here as it
should be delat with elsewhere.
SCUBA2: add check for SCAN data

Make sure this primitive only gets executed for SCAN data.
SCUBA2: perform steering checks on scan data

This recipe has to be able to process all types of data, regardless of
observation mode. A pointing steering primitive would be the ideal way
forward, but for now just use the existing primitive for scan data.