relayer should give error on non-flag arguments
I had a script that accidentally omitted the dashes on an argument. The expected behavior would be a failure with a message about the non-flag argument, but it was silently ignored. It was crazy confusing to debug, because flag arguments after a non-flag argument are ignored, so the error message was about not being able to connect to the wrong ETH endpoint even though the script error was on the `--log-level` flag.

We just need to apply the same fix to the relayer that was done for `swapd` in this PR:
https://github.com/AthanorLabs/atomic-swap/pull/311/files