{video,audio}decoder: don't raise an error in bus when no frames decoded

There's a sample in Khronos WebGL conformance test which is a webm container that expresses two streams:

Topology:
  container: WebM
    audio: Vorbis
    video: VP8

Properties:
  Duration: 0:00:05.038000000
  Seekable: yes
  Live: no
  Tags: 
      container format: Matroska
      language code: en

Nonetheless, the audio stream contains zero frames.

This configuration makes the audio decoder to raise an error in the bus. Using playbin2 the stream cannot be played at all. With playbin3 the stream is played and the error is raised at the end.

These error raising in audio decoding was added as a equivalence with video decoder in bug 711094, but even the proposal was questioned by the author.

This MR proposes a change in the audio decoder from raising an error in the bus, to a warning.

Questions:

1. Should be do the same in video decoding?
2. Should we add all these Khronos and WPT media samples in the integration tests? (Cc: @thiblahute)