core: improve log messages when we cannot process a mount point

Whenever we pick up a new line in /proc/self/mountinfo and want to
synthesize a new mount unit from it, let's say which one it is.
Moreover, downgrade the log message when we encounter a mount point with
an overly long name to LOG_WARNING, since it's generally fine to ignore
such mount points.

Also, attach a catalog entry to explain the situation further.

Prompted-By: #15221