cqlsh should return a non-zero error code if a query fails

patch by Branden Visser and Mikhail Stepura; reviewed by Mikhail Stepura for CASSANDRA-6344
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	bin/cqlsh
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt