Improve error message when browser denies access to getUserMedia()

Hi,

This is a follow-up discussion to 
https://code.google.com/p/chromium/issues/detail?id=257104

The WebRTC specification needs to provide a better error message in case 
the browser denies access to the camera, instead of the user. Currently, 
the specification states that PERMISSION_DENIED is the result of the 
user denying access. Instead of simply modifying the definition of 
PERMISSION_DENIED to cover both cases, I propose splitting it into 
separate cases:

PERMISSION_DENIED_BY_USER
PERMISSION_DENIED_BY_BROWSER

Alternatively, we need to add a String error message alongside the enum, 
and require browsers to issue a different error message for each case.

Thank you,
Gili