Fail better when input-code is interrupted
Fail better when input-code is interrupted

Hitting Control-C (which sends SIGINT) while we're waiting in the
readline-based input_code() function didn't shut down the process
properly: the reactor would wait for the readline thread to exit, which
wouldn't happen until it finished getting a code, which requires the
user to hit Return. I haven't found a good way to force the thread to
exit, or to synthetically inject a newline into stdin. So my compromise
is to tell the user that they need to hit Return to finish interrupting
the command.

See the _warn_readline() function for a list of other potential
approaches.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole