ResidueType: improving error messages for debugging
This makes it easier to know how things failed upon loading in a pose.
Former output example:
ERROR: unrecognized residue: Glc
New output examples:
ERROR: No match found for unrecognized residue at position 3
Looking for upper-terminal residue with 3-letter code: Glc
or:
ERROR: No match found for unrecognized residue at position 1
Looking for lower-terminal residue with 3-letter code: Glc
etc.