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.

src/network/wait-online/wait-online.c

@@ -223,6 +223,8 @@ static int run(int argc, char *argv[]) {
                                       "STATUS=Failed to wait for network connectivity...");
 
         r = sd_event_loop(m->event);
+        if (r == -ETIMEDOUT)
+                return log_error_errno(r, "Timeout occured while waiting for network connectivity.");
         if (r < 0)
                 return log_error_errno(r, "Event loop failed: %m");