UX: better rejection message when reply via email is too short

lib/email/processor.rb

@@ -52,6 +52,7 @@ module Email
                          when Email::Receiver::TopicNotFoundError          then :email_reject_topic_not_found
                          when Email::Receiver::TopicClosedError            then :email_reject_topic_closed
                          when Email::Receiver::InvalidPost                 then :email_reject_invalid_post
+                         when Email::Receiver::TooShortPost                then :email_reject_post_too_short
                          when Email::Receiver::UnsubscribeNotAllowed       then :email_reject_invalid_post
                          when ActiveRecord::Rollback                       then :email_reject_invalid_post
                          when Email::Receiver::InvalidPostAction           then :email_reject_invalid_post_action
@@ -69,6 +70,10 @@ module Email
         template_args[:post_error] = e.message
       end
 
+      if message_template == :email_reject_post_too_short
+        template_args[:count] = SiteSetting.min_post_length
+      end
+
       if message_template == :email_reject_unrecognized_error
         msg  = "Unrecognized error type (#{e.class}: #{e.message}) when processing incoming email"
         msg += "\n\nBacktrace:\n#{e.backtrace.map { |l| "  #{l}" }.join("\n")}"

lib/email/receiver.rb

@@ -31,6 +31,7 @@ module Email
     class TopicNotFoundError           < ProcessingError; end
     class TopicClosedError             < ProcessingError; end
     class InvalidPost                  < ProcessingError; end
+    class TooShortPost                 < ProcessingError; end
     class InvalidPostAction            < ProcessingError; end
     class UnsubscribeNotAllowed        < ProcessingError; end
     class EmailNotAllowed              < ProcessingError; end
@@ -932,7 +933,14 @@ module Email
       user = options.delete(:user)
       result = NewPostManager.new(user, options).perform
 
-      raise InvalidPost, result.errors.full_messages.join("\n") if result.errors.any?
+      errors = result.errors.full_messages
+      if errors.any? do |message|
+           message.include?(I18n.t("activerecord.attributes.post.raw").strip) &&
+           message.include?(I18n.t("errors.messages.too_short", count: SiteSetting.min_post_length).strip)
+         end
+        raise TooShortPost
+      end
+      raise InvalidPost, errors.join("\n") if result.errors.any?
 
       if result.post
         @incoming_email.update_columns(topic_id: result.post.topic_id, post_id: result.post.id)