Allow websocket routine with no arguments (and be less eager to CATCH)

[Altreus]

You currently have to do either

web-socket -> $incoming -> { }

or

web-socket -> $incoming, $on-close -> { }

As usual with my persistent bastardisations of your code, I don't actually use either thing in my test code, so I just had

web-socket -> { }

This caused the obscure error

  in code  at .../lib/Cro/WebSocket/Handler.pm6 (Cro::WebSocket::Handler) line 50

A runtime error from Perl caught by an overeager CATCH. First, it would be nice to support no arguments here; second, some errors should just die hard and I think ones from Perl6 itself should be included.