Improve error handling in compare-structures

CAD score computation requires residue numbers in model and reference
to match. This raises an error in the Scorer object otherwise.
Catch this early on and raise an error specific to compare-structures
action with instructions on how to get rid of it.

actions/ost-compare-structures

@@ -654,6 +654,11 @@ def _Main():
 
     args = _ParseArgs()
     try:
+        compute_cad = args.cad_score or args.local_cad_score
+        if compute_cad and not args.residue_number_alignment:
+            raise RuntimeError("Only support CAD score when residue numbers in "
+                               "model and reference match. Use -rna flag if "
+                               "this is the case.")
         reference = _LoadStructure(args.reference,
                                    sformat=args.reference_format,
                                    bu_idx=args.reference_biounit,