command/connect/proxy: ignore check doesn't exist on -register

[mitchellh]

This is reasonably jank and I'm open to other solutions, but its also simple.

If a check doesn't exist, then we get an error that sounds scarier than it is. This modifies the -register flag on the consul connect proxy subcommand to only output heartbeat errors if they aren't the "check doesn't exist" error. This error is common if the registration fails, but isn't a big deal in itself. We perform anti-entropy in the registration so that the check will always be recreated pretty quickly so this error is not important to us.

I also thought about only calling heartbeat() if register() is succeeding, but not sure if we want to maintain that state or not.

[banks]

Agree with assesment - somewhat jank but also not work a much more elaborate fix.

[mitchellh]

Agreed, let's just merge it. It just makes logs less loud.