Initial pass at fixing navigator.mediaDevices

[pjkundert]

First pass; chatty and probably wrong...

• Why didn't the final .catch do anything, and what is the right thing to do? Just log and re-raise the Error, and let the chips fall where they may? This is an un-recoverable error, since the caller must invoke the code from a secure context like https:// or file:///, so there's nothing we can do in the code to fix the situation. Might as well blow it up...
• The this._failed calls seem to blow up the SIP call, making the subsequent this.emit('getusermediafailed', error); signal sort of pointless; is the order wrong? Seems to be done in differing orders, elsewhere...

I'm not a Javascript expert, so probably other stuff, too.

fixes: #861

[pjkundert]

I have a demo that can serve up a simple JsSIP interface via a crude https session w/ self-signed certificates:

https://github.com/pjkundert/jsSip-demo