faet(ScreenObtainer): detect lack of permissions
faet(ScreenObtainer): detect lack of permissions

Both of the cases where user clicks cancel and when screen recording
permissions were not granted at the OS level are almost the same,
except for a small difference in the error.message.

This was only tested on Chrome 85.0.4183.121/Mac OS 10.15.6

jitsi/lib-jitsi-meet,jitsi/lib-jitsi-meet