* Fixes agent error handling when check definition is invalid. Distinguishes between empty checks vs invalid checks
* Made CheckTypes return Checks from service definition struct rather than a new copy, and other changes from code review. This also errors when json payload contains empty structs
* Simplify and improve validate method, and make sure that CheckTypes always returns a new copy of validated check definitions
* Tweaks some small style things and error messages.
* Updates the change log.
