ForceField tries to figure out why no residue matched a template

Here is an attempt at producing better error messages when ForceField can't match a template to a residue.  I kept it fairly simple for the moment.  It doesn't actually look at the bonds or try to identify common subgraphs.  It just counts the number of atoms of different types, and tries to offer a guess based on that and a few other simple things (like the residue name, and whether it's a terminal residue).