unit-name: generate a clear error code when converting an overly long fs path to...

unit-name: generate a clear error code when converting an overly long fs path to a unit name

(cherry picked from commit 9d5acfab20c5f1177d877d0bec18063c0a6c5929)
(cherry picked from commit 1579dce2c2a162bb09afb9a8a46fd4f7e8fbf1d5)
(cherry picked from commit 0488b743e9c6ab1e885933eebda4ba9232003a2a)

Gbp-Pq: Name unit-name-generate-a-clear-error-code-when-convertin.patch