transactions api in v5 will throw a generic 400 when the deadline parameter has expired

[cchighman]

I've found the transactions API to have a very consistent contract with strong validation until I ran into a scenario where I was only receiving "error: unable to process transaction" as a 400...nothing else. When i executed the OpenAPI example in the docs, the transactions api received a 200.

I had to work backwards comparing the two payloads until I landed here to realize the "deadline" parameter was involved. It would be helpful if explicit, clear validation was provided to enhance the user experience.