Signature Validation: Remove exceptions

[Ignacio Inglese (iNinja)]

Signature Validation: Remove exceptions

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Summary of the changes (Less than 80 chars)

Description

Part of the ValidateJWS/JWEAsync, removed exception throwing from signature validation. Refactored code and simplified some paths along the way.

[jennyf19]

[Franco Fung (FuPingFranco)]

LGTM