Use 550 code for unavailable actions

For actions that are either unimplemented or unavailable, send 550
instead of any other error code (eg. 450). Otherwise, some FTP clients
like Google Chrome appear to be broken.
Rudimentary UTF8 support

Advertise UTF8 support so FTP clients can interpret non-English file
names properly. Whether or not UTF8 is really supported is up to
implementation module.