fix: minor improvements to policies.yaml validation (datreeio#624)

* refactor: move extract errorPrefix to top of validatePoliciesYaml

* test: remove commented out test

* test: add test case for invalid json

* fix: adjust schema key in error message

* fix: add customRule index to error message

* refactor: rename function

* refactor: use pipe notation in jsonSchema field

* fix: duplicate defaultRules.yaml to 2 places (will be removed)

* refactor: move customRulesIdentifier validation logic to one iteration