Ignore invariant from code coverage

Author: benjie

src/execution/execute.ts

@@ -638,6 +638,7 @@ function handleFieldError(
     // In this mode, the client takes responsibility for error handling, so we
     // treat the field as if it were nullable.
   } else {
+    /* c8 ignore next 4 */
     invariant(
       false,
       'Unexpected errorBehavior setting: ' + inspect(exeContext.errorBehavior),