Expand Hotplug API to report unstable link events

This will become particularly handy to report link training failure.
Training failures occur when the display mode despite normally supported
fails to eventually succeed the training phase resulting in an unusable
display.
This change also includes a new error message for display setting
failure for undisclosed reason.

Bug: 374140876
Bug: 374138960
Test: manual
Change-Id: If6905978c2afab295ad9fa18439498a1ec7564f4