Cleanly reject message with no Signature header.
SignatureParametersParser throws SignatureParseException.
Verification refactor: extract signatureMatches()
Verification refactor: hasSignatureHeader() and signatureHeader()
Verification handles SignatureParseException.
SignatureParseException thrown for signature missing components.
KeyStore throws KeyStoreException for missing key.
Test partial signature (missing components) is rejected.
Cleanly reject message signed with key not in KeyStore.