Verify schema 'version' attribute

Verify that the schema 'version' attribute is the correct value. The
version attribute is used by Solr to determine how to parse the schema
so it is wise to make sure all schemas are using the same value.
Better schema error messages

Return schema validation errors as string messages rather than arbitrary
terms. This makes for cleaner error messages.
Merge pull request #323 from basho/misc/rz/verify-schema-version

Verify schema version