refactor: break message verification handling out into free function (#324)

* Better organise verification failures to make it easier to route based on variant

* Rename MessageVerificationError to MessageError for brevity

* Add handle_verification_issue to ActiveState

* Add handle_verification_issue to AwaitingResend state

* Add handle_verification_issue to AwaitingLogout and AwaitingLogon state

* Move handle_verification_failure out of session

Author: davidsteiner

crates/hotfix/src/message.rs

@@ -17,7 +17,7 @@ pub mod resend_request;
 pub mod sequence_reset;
 pub mod test_request;
 pub mod verification;
-pub mod verification_error;
+pub mod verification_issue;
 
 pub use parser::RawFixMessage;
 pub use resend_request::ResendRequest;