Allow per-call ExceptionSerializer

[TimMoore]

Many attributes of service calls can either be set globally at the service descriptor level, or specialized for each call in a service descriptor:

• request serializer
• response serializer
• circuit breaker
• auto-ACL

On the other hand, you can only configure a custom ExceptionSerializer (Java/Scala) globally for an entire service.

It would be useful to be able to configure a custom serializer for a specific call, falling back to the service-level custom serializer if there is one, or the Lagom default serializer otehrwise.