Improve error message when loaded sstable doesn't match schema

patch by slebresne; reviewed by driftx for CASSANDRA-6262
Merge branch 'cassandra-1.2' of http://git-wip-us.apache.org/repos/asf/cassandra into cassandra-1.2

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-1.2' into cassandra-2.0