JIRA: https://jira.spring.io/browse/INT-4503
Custom `ErrorHandler` can't be used if an external `RestTemplate`
is provided
* Replace `isClientSet()`` with ``!isClientSet()`` for `isTrue`
assertion in `errorHandler` method
* Replace assertion of `restTemplate` `isNull` with assertion
`!isClientSet()` `isTrue` in the `requestFactory()` method
* Add test coverage for the `errorHandler` into the `HttpDslTests`
**Cherry-pick to 5.0.x**
865e2162e5