onTtsEvent should null currentUtterance on tts error

onTtsEvent() should null out currentUtterance_ like 'interrupted'
and 'end' events do.  Otherwise, the call to startSpeakingNextItemInQueue_
will bail immediately since it thinks an utterance is still
speaking.

Bug: NONE
Test: cast on display assistant after TTS_EVENT_ERROR dispatched to chromevox
Change-Id: Iee5b35d763e526f85ace9d9bfcef85580ca61291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317986Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791960}

chrome/browser/resources/chromeos/accessibility/chromevox/common/tts_background.js

@@ -470,6 +470,7 @@ TtsBackground = class extends ChromeTtsBase {
         break;
       case 'error':
         this.onError_(event['errorMessage']);
+        this.currentUtterance_ = null;
         this.startSpeakingNextItemInQueue_();
         break;
     }