Make Channel.elems return a Failure

Rather than silently eating the currently available values on the
Channel and then returning the number of values eaten.

Inspired by discussion started at

  http://irclog.perlgeek.de/perl6/2016-09-14#i_13206149

src/core/Channel.pm

@@ -167,6 +167,10 @@ my class Channel {
         Nil
     }
 
+    method elems() {
+        Failure.new("Cannot determine number of elements on a {self.^name}")
+    }
+
     method fail($error is copy) {
         $!closed = 1;
         $error = X::AdHoc.new(payload => $error) unless nqp::istype($error, Exception);