Add more explicit error message.

pac.print does not work in PAC/MCE equations.

matlab/print_expectations.m

@@ -93,6 +93,7 @@ end
 expectationmodel = M_.(expectationmodelfield).(expectationmodelname);
 
 % Get the name of the associated VAR model and test its existence.
+if isfield(M_.(expectationmodelfield), 'auxiliary_model_type')
     if ~isfield(M_.(expectationmodel.auxiliary_model_type), expectationmodel.auxiliary_model_name)
         switch expectationmodelkind
           case 'var'
@@ -102,6 +103,9 @@ if ~isfield(M_.(expectationmodel.auxiliary_model_type), expectationmodel.auxilia
           otherwise
         end
     end
+else
+    error('print method does not work in PAC/MCE.')
+end
 
 auxmodel = M_.(expectationmodel.auxiliary_model_type).(expectationmodel.auxiliary_model_name);