Error Handler Flow will get invoked when Connector is unable to connect to external system as mentioned like providing wrong URL or Service is unavailable. When Service is available and returns wrong response this flow shouldn't get called.
Error handler flow
Optional. Identify a flow rule to route work items that cannot continue in its current flow because this connector rule was unable to connect to the external system. The default flow rule Work-.ConnectionProblem provides one approach for handling such problems. See Handling connector exceptions.