Updated error code for incorrect recipients in GhostMailer

no issue

- By default, GhostMailer throws EmailError with statusCode as `500` for any failure in sending mail
- In case of failure due to `RecipientError`, status code as now correctly sent as `400` as its a bad request and not an error we can't handle.

core/server/services/mail/GhostMailer.js

@@ -48,9 +48,11 @@ function createMessage(message) {
 
 function createMailError({message, err, ignoreDefaultMessage} = {message: ''}) {
     const fullErrorMessage = defaultErrorMessage + message;
+    let statusCode = err.name === 'RecipientError' ? 400 : 500;
     return new errors.EmailError({
         message: ignoreDefaultMessage ? message : fullErrorMessage,
         err: err,
+        statusCode,
         help: helpMessage
     });
 }