Fixed error message for zwitterion residues

Previously, pdb2gmx complained that the residue was
not available as a 'starting' terminus for isolated
residues (usually zwitterions). This is really confusing
when starting terminus residuetypes ARE available. Now
we properly explain there is no type to use the residue
isolated.

Change-Id: I0c9c8328f12e75a5a7ffaa8ea894ff46f8fdfe75
Merge "Fixed error message for zwitterion residues" into release-4-6