fix(AWS Local Invocation): Fix invalid result handling

Author: medikoo

lib/plugins/aws/invokeLocal/index.js

@@ -799,7 +799,17 @@ class AwsInvokeLocal {
       if (result instanceof Error) {
         handleError.call(this, result);
         return;
-      } else if (result.headers && result.headers['Content-Type'] === 'application/json') {
+      }
+      try {
+        JSON.stringify(result);
+      } catch (error) {
+        throw new ServerlessError(
+          `Function returned invalid (not a JSON stringifiable) value: ${inspect(result)}`,
+          'INVALID_INVOKE_LOCAL_RESULT'
+        );
+      }
+
+      if (result.headers && result.headers['Content-Type'] === 'application/json') {
         if (result.body) {
           try {
             Object.assign(result, {