MAC spoofing failure notification should point to the corresponding documentation

It was implemented initially, and then the move to Wheezy made the link invisible (#7249 (closed)), and for Jessie #7989 (closed) lead us to remove it. It would be great to tell the user what to do when MAC spoofing fails.

As the blueprint linked above points out:

• a desktop notification may not be the best way to convey this message;
• a process started in the background in a udev hook may not be the best way to trigger that notification.

So this needs both UX and implementation research, quite possibly done together (this is a corner case, and we don’t want to design something perfect that’s so hard to implement that it’ll never be done).

Related issues

• Related to #7249 (closed)
• Related to #7989 (closed)
• Related to #14544
• Related to #16260
• Blocked by #16385 (closed)

Original created by @intrigeri on 10559 (Redmine)