UX: Improve error messages for minimum and maximum username lengths.

lib/validators/max_username_length_validator.rb

@@ -0,0 +1,19 @@
+class MaxUsernameLengthValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(value)
+    return false if value < SiteSetting.min_username_length
+    @username = User.where('length(username) > ?', value).pluck(:username).first
+    @username.blank? ? true : false
+  end
+
+  def error_message
+    if @username.blank?
+      I18n.t("site_settings.errors.max_username_length_range")
+    else
+      I18n.t("site_settings.errors.max_username_length_exists", username: @username)
+    end
+  end
+end