Add more explicit message for format validation errors

app/models/user.rb

@@ -63,7 +63,7 @@ class User < ApplicationRecord # rubocop:disable Metrics/ClassLength
   validates :email, fake_email: true, typoed_email: true
   validates :screen_name,
             presence:    true,
-            format:      { with: SCREEN_NAME_REGEX },
+            format:      { with: SCREEN_NAME_REGEX, message: I18n.t("activerecord.validation.user.screen_name.format") },
             length:      { minimum: 1, maximum: 16 },
             uniqueness:  { case_sensitive: false },
             screen_name: true

config/locales/activerecord.en.yml

@@ -96,6 +96,10 @@ en:
         primary_color: "The primary/brand colour of the site, used for navigation, links, etc."
         success_color: "Colour used for messages if something went through successfully."
         warning_color: "Colour used for warnings if something non-critical has happened."
+    validation:
+      user:
+        screen_name:
+          format: "contains invalid characters"
   helpers:
     submit:
       user: