Added proper serialization/deserialization exception propagation

Current implementation causes incorrect handling by ServiceStack of   web responses (ex.: 401 Authorization required) when performing async calls. ServiceStack expects a SerializationException - not a ProtoBuf.ProtoException. Wrapping the exception in a SerializationException fixes this incorrect behavior.
Exception messages are more explicit.
Merge pull request #817 from PilotMartin/master

Fix for async exceptions during deserialization: added proper serialization/deserialization exception propagation