Think about validating inputs in keygen_with_dealer and not just in generate_secret_polynomial

[natalieesk]

Currently we handle validation of min_signers and max_signers in generate_secret_polynomial. When calling keygen_with_dealer we accept these inputs but perform no validation on them until generate_secret_polynomial is called. generate_secret_polynomial is public and so if it's directly accessible we probably would want to validate the inputs in that function however. We should consider how best to handle validation in this instance as we would reduce computation effort by performing those validations up front.

[conradoplg]

Done in #453 after the audit found an issue related to this (good job anticipating it 😆 )