The error event on `<source>` elements doesn't work. Looking at #4370, it seems like there is an `onError` for `<video>` but I can't tell if that's supposed to work for `<source>` too.

The error event is useful so I can deduce which format the browser is using.

var parentTag;
    if (hostParent != null) {
      namespaceURI = hostParent._namespaceURI;
      parentTag = hostParent._tag;
    } else if (hostContainerInfo._tag) {
      namespaceURI = hostContainerInfo._namespaceURI;
      parentTag = hostContainerInfo._tag;
    }
    if (namespaceURI == null ||
        namespaceURI === DOMNamespaces.svg && parentTag === 'foreignobject') {
      namespaceURI = DOMNamespaces.html;
    }
    if (namespaceURI === DOMNamespaces.html) {
      if (this._tag === 'svg') {
        namespaceURI = DOMNamespaces.svg;
      } else if (this._tag === 'math') {
        namespaceURI = DOMNamespaces.mathml;
      }
    }
    this._namespaceURI = namespaceURI;
    if (__DEV__) {
      var parentInfo;
      if (hostParent != null) {
        parentInfo = hostParent._ancestorInfo;
      } else if (hostContainerInfo._tag) {
        parentInfo = hostContainerInfo._ancestorInfo;
      }
      if (parentInfo) {