using stop cycle on top of the logs is a suitable way to prevent MWG from logging specific requests. When I want to exclude something I usually do it the same way :-)
Probably it would be an additional benefit to file an SR with support and analyze why the error is thrown so often?
Maybe it would be another option to add a stop ruleset rule to the coaching rule set that disabled coaching for the user-agents that fail. You don't need to skip log file writing for errors that don't happen.