THRIFT-2854 Go Struct writer and reader looses important error information

This PR fixes https://issues.apache.org/jira/browse/THRIFT-2854.
The included test will cover the complete client error handling code except the check for the Sequence ID. (unless I miss something) I will submit a follow up PR to test the error handling for the Sequence ID too once this PR has been merged. The covered code makes up the majority of the server error handling code too.

And yes.. it was not fun to code the test.. :P