Warn about lack of WebRTC

[inputsh]

1. Can you describe the problem and bug in more detail?

I had WebRTC disabled, tried pairing new device, and the QR code wasn't being generated.

2. How can we replicate the issue?

• Have WebRTC disabled in a browser. For example, use this plugin: https://addons.mozilla.org/en-US/firefox/addon/happy-bonobo-disable-webrtc/
• Install Memex Go.
• Sync → Pair new device.
• Reach the stage in which it tries to generate a QR code.
• Nothing happens. Only when I looked in my console did I see what went wrong.

3. Expected behavior (i.e. solution)

• Check for WebRTC support.
• If not present, display a meaningful error message instead of an infinite spinner.

4. Error stack (from extension crash page)

This was shown in my console:

Uncaught (in promise) d: No WebRTC support: Not a supported browser

Can't find more meaningful logs than that.