Routing on exception

This is a utility module that adds the ability to specify actions that should be taken
on your route when an HttpException occurs on it. A common example would be, when an
access check fails and an AccessDeniedException is returned.

Obviously you can write your own EventSubscriber to handle your case, but I think for
common/straightforward actions (i.e. redirecting the user on an access denied response),
declaring it alongside the route is clearer.

Example - CSRF tokens

One example is if you want to add a CSRF Token to your route, but you'd rather the user
was redirected if this validation fails, rather than being left on an Access Denied page.

Your my_module.routing.yml may look like this:

    my_module.listing_route:
      path: '/my/listing/path'
      defaults:
        _controller: '\Drupal\my_module\Controller\CoolController::listThings'
      methods: [ GET ]

    my_module.action_route:
      path: '/my/listing/path/take_action'
      defaults:
        _controller: '\Drupal\my_module\Controller\CoolController::takeAction'
      methods: [ GET ]
      options:
        on_exception:
         - on:
             - name: '.*AccessDeniedHttpException$'
               message: '.*csrf_token.*'
           then:
             - log: 'Oh noes, this should not happen but it did.'
             - redirect: my_module.listing_route
        requirements:
          _csrf_token: 'TRUE'

Or you could redirect all exceptions regardless of what they are:

    my_module.action_route:
      path: '/my/listing/path/take_action'
      defaults:
        _controller: '\Drupal\my_module\Controller\CoolController::takeAction'
      methods: [ GET ]
      options:
        on_exception:
          - then:
              - redirect: my_module.listing_route

Rules

For an on_exception rule to match at least one of its 'on' conditions must match all
of its conditions.

More than one rule can be specified for the same route. The first rule that matches
will have all of its actions executed.

Todo

If others find this helpful, we should probably refactor this module to use proper
Condition and Action Plugins rather than all the switch statements.