Fix handling of missing Endpoint for client communication

The `getEndpointForClient()` threw an `InvalidStateException`if no Endpoint could be found for the Client. However, this exception had not been caught or otherwise been handled apropriately by any code yet. Thus I changed this to throw a `ResouceAccessException`instead which is properly handled.
Yes, you're right. I originally introduced the JavaDoc only to document the fact, that a ResourceAccessException is thrown. However, since it is actually thrown by `getEndpointForClient()` it doesn't make much sense now ... I will remove the JavaDoc completely ...