Messages permanently retry when the log group doesn't exist

[dev-guy]

When the ensureLogGroup option is falsy, attempting to log a message causes it to be retried indefinitely.

For example, I set up an error handler that logs to the console. After attempting to log only one message, the handler outputs this until I either stop the transport or kill the process:

ResourceNotFoundException: The specified log group does not exist

[terrisgit]

This might be fixed by #184