Made errorFrame an interface that can implemented by the different types of errors from a cassandra server.
Moved errorFrame and err constants to errors.go, added (*frame.)readError() errorFrame function
Updated conn.go to use new error syntax
Added test to ensure library can properly decode error response from cassandra
Expose error structs
Merge pull request #161 from ChannelMeter/error_response_refactor

Refactor ErrorFrame response in order to expose ErrorFrame message bodies.