Emulator does not throw error when AppSync does

[ScottAP108]

Testing locally, I added a scanIndexForward property to a mapping template for a scan operation, not realizing at the time that this property is only permitted in queries. The emulator did not throw an error for this, but once I deployed, AppSync did.

The emulator seems to simply ignore extraneous properties in mapping templates that are not necessary to construct the params for the dynamo request. If the goal is to faithfully emulate the behavior of AppSync as closely as possible, maybe it would be best to have a list of permissible properties for each operation, and throw an error when a property outside of that list is encountered.