[ck2cti] Provide better error message for reactions with missing data

For falloff / chemically activated reactions (with "(+M)" appearing in the
reaction equation), check that the alternate rate expression has been
provided. Previously, this would only generate an inscrutable error when
importing the XML file.

interfaces/cython/cantera/ck2cti.py

@@ -1499,6 +1499,10 @@ class Parser(object):
             reaction.kinetics = ThirdBody(arrheniusHigh=arrhenius,
                                           parser=self,
                                           efficiencies=efficiencies)
+        elif reaction.thirdBody:
+            raise InputParseError('Reaction equation implies pressure '
+                'dependence but no alternate rate parameters (i.e. HIGH or '
+                'LOW) were given for reaction {0}'.format(reaction))
         else:
             reaction.kinetics = arrhenius