Fix joining call without media devices

Since MediaDevicesSource was introduced errors thrown when requesting
the media streams were wrongly swallowed; those errors could be real
errors, like the camera being used by another program, but they would
happen too if both audio and video devices were explicitly disabled. As
the errors were not propagated joining the call failed, because
LocalMedia assumed that a stream was available but ended adding a null
stream instead.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>