[RACL] Improve error checking when parsing a RACL config

[Razer6]

#25664 adds the initial part of the RACL implementation. This confidently parses the RACL config without much error checking. This should be improved over time but should allow the functional implementation to continue.

          I'd strongly suggest putting a tiny bit more type checking in here. Something like extracting `roles` and `policies` as a list / dictionary, respectively (doing a basic type check) and maybe checking the item and key/value types for them respectively.

Extracting that properly will get rid of the dictionary lookups lower down in the function.

Originally posted by @rswarbrick in #25664 (comment)