It's possible to specify contradictory caveats

Presently when using a `CaveatBuilder` or the declaratory `Caveats` interface in `createDelegation`, it's possible to specify caveats that are contradictory - for instance `timestamp` caveats that specify non-overlapping valid periods.

It could be useful for a caller to receive some sort of feedback about contradictory caveats. This could be:

- `validateCaveats` utility function
- error thrown from `CaveatBuilder.build()` or `createDelegation`

It could be helpful to parse the parent delegations if they exist (although if the parent delegation is a non-leaf, then it's less useful, and might provide a false sense of security).

Start by tracing CaveatBuilder, the declaratory Caveats interface, and createDelegation to understand how caveats are assembled and where validation could occur. Compare the proposed validateCaveats utility and build/createDelegation errors, including the limitations of checking parent delegations. Done means the behavior for contradictory caveats is decided and implemented with coverage for the selected entry points.

Written by the indexing model from the issue text.