don't try to resolve/download SIFTS records for structures not in PDB

Jalview will report exceptions when a PDB or mmCIF file associated with a sequence is viewed if the file has a non-empty 'ID' field (as returned via getTitle() by the jmol Data API).

These have no impact on functionality, but are most likely disconcerting for users!

There's also the possibility of ID collision (when a PDB record exists for an ID field but it is a different structure to the one being imported). Suggest creating another issue to address this problem.