wait-online: improve timeout log message

The time-out when waiting to reach the online state is a pretty
regularly seen error, let's print an explicit log message for it. The
previous "Event loop failed: timed out" message is a bit too low-level I
think for regular users (as event loops are a developer's concept, not a
user's, really).

Note that outputting low-level error messages is generally actually OK I
think — for unexpected errors, but this timeout is a pretty expected one,
directly configurable by the user, hence output something friendly.