Generate more informative error message if residue template not found (#3558)

* Fix #3557 : Generate more informative error message if residue template not found.

* Fix test failures

wrappers/python/openmm/app/forcefield.py

@@ -1086,12 +1086,12 @@ class ForceField(object):
         # Find the template matching each residue, compiling a list of residues for which no templates are available.
         bondedToAtom = self._buildBondedToAtomList(topology)
         templates = list() # list of templates matching the corresponding residues
-        for res in topology.residues():
+        for residue in topology.residues():
             # Attempt to match one of the existing templates.
-            [template, matches] = self._getResidueTemplateMatches(res, bondedToAtom, ignoreExternalBonds=ignoreExternalBonds)
+            [template, matches] = self._getResidueTemplateMatches(residue, bondedToAtom, ignoreExternalBonds=ignoreExternalBonds)
             # Raise an exception if we have found no templates that match.
             if matches is None:
-                raise ValueError('No template found for residue %d (%s).  %s' % (res.index+1, res.name, _findMatchErrors(self, res)))
+                raise ValueError('No template found for chainid <%s> resid <%s> resname <%s> (residue index within topology %d).\n%s' % (residue.chain.id, residue.id, residue.name, residue.index, _findMatchErrors(self, residue)))
             else:
                 templates.append(template)