Consider renaming all isValidXYZFeature to checkValidXYZFeature within psa token library

[yogeshbdeshpande]

This issue is a twin issue to the original issue reported in ccatoken repository:

veraison/ccatoken#4

Basically, if the Claim checker functions are named isValidXYZFeature then they should return a boolean.

Otherwise, they should be renamed as checkValidXYZFeature() which would return an error in case any checks fail!