Exception type heirarchy

Currently, many error conditions are thrown as `Exception`.
Instead they should be made more specific, all inheriting from a central library-specific base exception.
See https://github.com/99designs/http-signatures-ruby for reference.

Review the linked Ruby library for its exception hierarchy. Examine the codebase for where `Exception` is thrown, likely in classes like `Signer` or `Verifier`. Create a base exception class (e.g., `HttpSignaturesException`) and specific subclasses for different error conditions. Update throw statements and ensure existing tests still pass.

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