Handle network error on synthesis

[compulim]

Today, when network error happened while synthesizing, it will not emit and "error" event.

The correct behavior should be "start" event followed by "error" event.

Network error

Testing against server-based voice.

• Speak an utterance
• Expect: receive start event
• (A few seconds later)
• Expect: receive error event of type SpeechSynthesisErrorEvent
  • error field set to "synthesis-failed"
  • utterance field set to the `SpeechSynthesisUtterance instance
  • elapsedTime field is set
• There will be no end event