Fix #4165: Add mechanism to test error responses.

https://danbooru.donmai.us/posts.json?error=500&message=testing

app/controllers/application_controller.rb

@@ -10,6 +10,7 @@ class ApplicationController < ActionController::Base
   before_action :api_check
   before_action :set_variant
   before_action :enable_cors
+  before_action :cause_error
   after_action :reset_current_user
   layout "default"
 
@@ -107,6 +108,19 @@ class ApplicationController < ActionController::Base
     request.variant = params[:variant].try(:to_sym)
   end
 
+  # allow api clients to force errors for testing purposes.
+  def cause_error
+    return unless params[:error].present?
+
+    status = params[:error].to_i
+    raise ArgumentError, "invalid status code" unless status.in?(400..599)
+
+    error = StandardError.new(params[:message])
+    error.set_backtrace(caller)
+
+    render_error_page(status, error)
+  end
+
   User::Roles.each do |role|
     define_method("#{role}_only") do
       if !CurrentUser.user.send("is_#{role}?") || CurrentUser.user.is_banned? || IpBan.is_banned?(CurrentUser.ip_addr)