To do this, you must have some coding ability, which I'm hoping you do since you're on the developer forums. Are you using the signup form post URL described here? If so, we send you an HTTP response from our server with the error code, (400 or 429 in the case of a failure), and an error message body. That error message body is what you're looking to display to your end user who has input their email address. Your code must capture our HTTP error response body, and display it to the user on your failure page.
If you're using a plugin, you can contant the plugin developer, who should be listed wherever you downloaded the plugin to request this feature.
Partner API Support Engineer
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