Make helper.getExceptionMessage synchronous

We now have description of an exception, no need for a roundtrip
to the backend.

lib/helper.js

@@ -25,24 +25,13 @@ class Helper {
   }
 
   /**
-   * @param {!Connection} client
    * @param {!Object} exceptionDetails
    * @return {string}
    */
-  static async getExceptionMessage(client, exceptionDetails) {
-    let message = '';
-    let exception = exceptionDetails.exception;
-    if (exception) {
-      let response = await client.send('Runtime.callFunctionOn', {
-        objectId: exception.objectId,
-        functionDeclaration: 'function() { return this.message; }',
-        returnByValue: true,
-      });
-      message = response.result.value;
-    } else {
-      message = exceptionDetails.text;
-    }
-
+  static getExceptionMessage(exceptionDetails) {
+    if (exceptionDetails.exception)
+      return exceptionDetails.exception.description;
+    let message = exceptionDetails.text;
     if (exceptionDetails.stackTrace) {
       for (let callframe of exceptionDetails.stackTrace.callFrames) {
         let location = callframe.url + ':' + callframe.lineNumber + ':' + callframe.columnNumber;