Better error message when firefox can't start webrtc

[mscherer]

If one disable webrtc (about:config => media.peerconnection.enabled set to false), galene will silently fail with a error in the web console:

Uncaught (in promise) ReferenceError: RTCPeerConnection is not defined
    newUpStream https://galene.example.org/protocol.js:406
    newUpStream https://galene.example.org/galene.js:779
    addLocalMedia https://galene.example.org/galene.js:1066

This setting is documented on https://wiki.mozilla.org/Media/WebRTC/Privacy

While that's not a default configuration, some people disable Webrtc for privacy (with https://addons.mozilla.org/en-US/firefox/addon/happy-bonobo-disable-webrtc/ , and afaik, the Tor browser do it).

[jech]

Fixed in e04193f