old_contents: from flask import render_template, request, jsonify
  from . import main
  
  
  @main.app_errorhandler(403)
  def forbidden(e):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'forbidden'})
          response.status_code = 403
          return response
      return render_template('error/403.html'), 403
  
  
  @main.app_errorhandler(404)
  def page_not_found(e):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'not found'})
          response.status_code = 404
          return response
      return render_template('error/404.html'), 404
  
  
  @main.app_errorhandler(500)
  def internal_server_error(e):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'internal server error'})
          response.status_code = 500
          return response
      return render_template('error/500.html'), 500
  
new_contents: from flask import render_template, request, jsonify
  from . import main
  
  
  @main.app_errorhandler(403)
  def forbidden(e=None):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'forbidden'})
          response.status_code = 403
          return response
      return render_template('error/403.html'), 403
  
  
  @main.app_errorhandler(404)
  def page_not_found(e=None):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'not found'})
          response.status_code = 404
          return response
      return render_template('error/404.html'), 404
  
  
  @main.app_errorhandler(500)
  def internal_server_error(e=None):
      if request.accept_mimetypes.accept_json and \
              not request.accept_mimetypes.accept_html:
          response = jsonify({'error': 'internal server error'})
          response.status_code = 500
          return response
      return render_template('error/500.html'), 500
  
subject: fix exceptions