Add option to error on scan instead of defaulting to scan when an index is not found

[tlperkins]

Queries should not default to scanning which would break the bank and not perform well in production.

[devinivy]

The queries do not default to scan in the current version on master. They actually use indices when indices are available. See this section of the readme: https://github.com/gadelkareem/sails-dynamodb#using-dynamodb-indexes