Standardize validation error text across target languages?

[robin-aws]

Spin-off from #582 (comment)

Polymorph generally evaluates constraints "on the client side", i.e. in the target language rather than the source language. That means unlike with a real microservice, the exact error messages can be different.

On the one hand, we've seen any variation can be cause for alarm when trying to assert that a polymorphed library behaves identically to the original: #362

On the other hand, having the validation in each target language means the error text can be slightly more tuned to that language, which is better UX: "The full_name field must be set - call .set_full_name(...) on the builder" (using e.g. Rust terms and concrete symbols) as opposed to "The FullName member must be present" (generic validation on the server side, has to use generic Smithy terms)

[robin-aws]

Relevant: Smithy now includes a built-in error shape for validation exceptions, smithy.framework#ValidationException. See https://smithy.io/2.0/tutorials/full-stack-tutorial.html#generating-the-server-sdk