Instead of failing with an error if another operation is in progress, which is
impossible to know as a user, chain them using the primises API.

CHROME_EXTENSION_USER_GESTURE_REQUIRED will be triggered if
the extension installation fails, because the request to get desktop
stream did not originate in user gesture handler (a button click for
example).

Firefox uses "constraint" as the key in the error:

```
[modules/RTC/RTCUtils.js] <value/<>:  Failed to get access to local media. Error
MediaStreamError { name: "OverconstrainedError", message: "Constraints could be
not satisfied.", constraint: "width", stack: "" } Object { audio: false, video:
Object }  lib-jitsi-meet.min.js:6:10419
```

#416

When calling getUserMedia in chrome for just video with no video devices connected we receive error with name DevicesNotFoundError.