dispatch: handle late KeyboardInterrupt occurred in run()
User can press Ctrl+C while flushing streams in dispatch.run(). In such case,
I think exiting with 255 is better than printing Python traceback and exiting
with 1.
pub type ExitCode = i32;
/// Successful exit
pub const OK: ExitCode = 0;
/// Generic abort
pub const ABORT: ExitCode = 255;
/// Command not implemented by rhg
pub const UNIMPLEMENTED_COMMAND: ExitCode = 252;