application_controller.rb

class ApplicationController < ActionController::Base
  rescue_from StandardError, with: :error500

  def error500
    respond_to do |format|
      format.json { render json: {message: 'Internal Server Error'}, status: :internal_server_error }
      # ↓こいつのContent-Typeをtext/htmlに固定したい
      format.any { render 'error500', formats: :html, status: :internal_server_error }
  end
end