_FTPERROR returns the error code of the last error reported by FTP. Some of the possible codes are:
| 101 | You cannot change the remote host at this time |
| 102 | The remote host address is invalid |
| 143 | Busy performing current action |
| 145 | Can't open local file for reading |
| 146 | No remote file specified while uploading |
| 311 | Accept failed for data connection |
| 312 | Asynchronous select failed for data connection |
| 11002 | Non-authoritative 'Host not found' |
| 11003 | Non-recoverable errors: FORMERR, REFUSED, NOTIMP |
| 11104 | Valid name, no data record (check DNS setup) |