Provide better error message when pluggable REST API can't reach service due to version mismatch

It's always been the case that the pluggable REST API (i.e. the REST API requests that begin with _p) will only get proxied to a service on a different node if that service is running on the same version as the proxy node. (E.g. MB-46626.)

When it happens that the service is present in the cluster but is running the wrong version we should return an error indicating this. This will mean pluggable UI writers will be able to explain to users what is going on; namely that the pluggable UI is not available on this node, but may be available on a different node.