Tighten integration with Ruby exceptions by tying in Exception#message

Tighten integration with Ruby exceptions by tying in Exception#message

For Thrift exceptions that contain only a single string field, integrate
these even more tightly with Ruby exceptions by aliasing Ruby's
Exception#message field with the Thrift field.

Ruby exception objects created by Thrift will now display properly in
Ruby backtraces, etc. without any special handling by client code.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665503 13f79535-47bb-0310-9956-ffa450edef68