Update some entity processing errors to 400.28

[ktuite]

PR #844 introduces a new Problem Problem.user.invalidEntity (400.28) to return when creating an entity from JSON fails because of some formatting or validation problem. New code around JSON entity extraction uses this problem.

Many of the previous validation error cases throw Problem.user.entityViolation (409.14), which is a conflict problem rather than a bad request problem. This problem was made back when we started parsing submission XML to create entities and foresaw entity conflict as one possible problem.

I think we could go back and update pure validation errors to throw the 400.28 error instead of 409.14. The names could also be better - entityConflict and invalidEntity or something.

[matthew-white]

I also think we should try to use more specific Problems that already exist when those are appropriate. For example, Problems like Problem.missingParameter() or Problem.invalidDataTypeOfParameter() that have well-defined detail properties. I think it's nice to have Problem.invalidEntity() as a fallback, but I feel like our first choice should be to reach for something more specific.