clarify that a formula needs to be added to do anything in storm-pomdp

src/storm-pomdp-cli/storm-pomdp.cpp

@@ -204,8 +204,12 @@ int main(const int argc, const char** argv) {
                 STORM_PRINT_AND_LOG(" done." << std::endl);
             }
 
+        } else {
+            STORM_LOG_WARN("Nothing to be done. Did you forget to specify a formula?");
         }
 
+
+
         // All operations have now been performed, so we clean up everything and terminate.
         storm::utility::cleanUp();
         return 0;