ErrorEntity should be Serializable

[haqer1]

ErrorEntity should be Serializable to avoid exceptions like the following in environments involving some remoting:
com.arangodb.ArangoDBException : Response: 409, Error: 1210 - unique constraint violated - in index 1355495 of type hash over ["email"]; conflicting key: 1355503 [Proxied because : Original exception caused: class java.io.NotSerializableException: com.arangodb.entity.ErrorEntity]

[mvollmary]

Thanks for the feedback. Change will be in the next release.