FIX: Unreachable .cvmfswhitelist Kills `cvmfs_server list`

The function `check_expiry()` in `cvmfs_server` called `die()` when the whitelist expiry check failed (i.e. the whitelist was not available). This is bad, since it truncates the output of `cvmfs_server list` for example. Now it only prints a message to **stderr** instead of interrupting the processing of the whole script.