Consolidate stoichiometry checks

...and change an error type.
Co-Authored-By: Jackson Burns <jwburns@mit.edu>

Co-authored-by: Jackson Burns <33505528+JacksonBurns@users.noreply.github.com>

Author: rwest

rmgpy/data/kinetics/family.py

@@ -2708,18 +2708,15 @@ def get_labeled_reactants_and_products(self, reactants, products, relabel_atoms=
                 template_reactants = [x.item for x in template.reactants]
         else:
             template_reactants = [x.item for x in template.reactants]
-
+        if (num_reactants := len(reactants0)) != (num_template := len(template_reactants)):
+            raise ValueError(f"Reaction has {num_reactants} reactants but template has {num_template} reactants.")
         if len(reactants0) == 1:
-            if len(template_reactants) != 1:
-                raise ValueError(f"Reaction has {len(reactants0)} reactants but template has {len(template_reactants)} reactants.")
             molecule = reactants0[0]
             mappings = self._match_reactant_to_template(molecule, template_reactants[0])
             mappings = [[map0] for map0 in mappings]
             num_mappings = len(mappings)
             reactant_structures = [molecule]
         elif len(reactants0) == 2:
-            if len(template_reactants) != 2:
-                raise ValueError(f"Reaction has {len(reactants0)} reactants but template has {len(template_reactants)} reactants.")
             molecule_a = reactants0[0]
             molecule_b = reactants0[1]
             # get mappings in forward direction
@@ -2734,8 +2731,6 @@ def get_labeled_reactants_and_products(self, reactants, products, relabel_atoms=
             reactant_structures = [molecule_a, molecule_b]
             num_mappings = len(mappings_a) * len(mappings_b)
         elif len(reactants0) == 3:
-            if len(template_reactants) != 3:
-                raise ValueError(f"Reaction has {len(reactants0)} reactants but template has {len(template_reactants)} reactants.")
             molecule_a = reactants0[0]
             molecule_b = reactants0[1]
             molecule_c = reactants0[2]
@@ -2750,7 +2745,7 @@ def get_labeled_reactants_and_products(self, reactants, products, relabel_atoms=
             reactant_structures = [molecule_a, molecule_b, molecule_c]
             num_mappings = len(mappings_a) * len(mappings_b) * len(mappings_c)
         else:
-            raise NotImplementedError('You have {0} reactants, which is unexpected!'.format(len(reactants)))
+            raise ValueError('You have {0} reactants, which is unexpected!'.format(len(reactants)))
 
         for mapping in mappings:
             try: