Fix #32: provide 426 responses on data channel "exceptional" error events.
Fix #32: provide 426 responses on data channel "exceptional" error events.

- When DTPHandler.handle_expt is called a 426 response is now provided.
- When DTPHandler.handle_error is called a 426 is now provided among a detailed message response describing what went wrong. Those message occurs in case of exceptions affecting connection or file descriptors dragged in data transfer (e.g. "No space left on device")

giampaolo/pyftpdlib