Validate that throws clauses in Thrift contain only exceptions

Validate that throws clauses in Thrift contain only exceptions

Summary: Throwing non-exceptions, though allowed in some languages, is a weird concept and causes problems in many places. Disallow it in the Thrift compiler and throw an informative error.

Reviewed By: dreiss

Test Plan: Generate a file with an int or something in a throws clause. Peep the nice informative compiler error.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665450 13f79535-47bb-0310-9956-ffa450edef68