Add assertion on returning with exceptions.

By checking for the error state before the chosen CF call, we
know that it's not the CF call that set the error state.

src/runtime/objmodel.cpp

@@ -4142,6 +4142,10 @@ Box* callCLFunc(FunctionMetadata* md, CallRewriteArgs* rewrite_args, int num_out
         }
     }
 
+    // We check for this assertion later too - by checking it twice, we know
+    // if the error state was set before calling the chosen CF or after.
+    ASSERT(!PyErr_Occurred(), "");
+
     Box* r;
     // we duplicate the call to callChosenCf here so we can
     // distinguish lexically between calls that target jitted python