Change information line logged by Runner while unregistering

While executing the unregister command, Runner is logging such line:

Deleting runner...

This may provide to misunderstanding what unregister command is for. We should change this line to:

Unregistering runner from GitLab

and also change the error message from:

FATAL: Failed to delete runner

to

FATAL: Failed to unregister runner

Successor of #2164 (closed)

changed milestone to %v9.1

added ~1009246 label

mentioned in merge request !540 (merged)

closed via merge request !540 (merged)