Do not tolerate quite so many bind failures

Our current code in arti warns when we fail to bind a given port, but only exits if we fail to bind every port of a given type.

That's reasonable when we fail to bind to [::1]:9051 because we have no IPv6 support. But if we're told to bind to 9051 and we fail on [::1]:9051 with EADDRINUSE, then maybe we should treat that as a fatal error.