Better handling for error conditions

[piegamesde]

At the moment, our user-facing error handling is greatly lacking. The following common error cases should be handled more gracefully:

• Wrong code entered
• "crowded" server error (too many people using that code at once)
• Peer abort
• File not accepted by peer

The error handling should:

• Write an error message to the other side and gracefully close the connection
• Let the other side know if the crypto went wrong

[piegamesde]

I think most of the error handling is quite okay now. Open new issues if specific problems show up.