added message for permission denied exceptions

[marxide]

I found myself wanting to pass a string to the PermissionDenied exceptions to that I could display more information to the user on the 403 page.

This PR adds an attribute permission_denied_message and an overridable method get_permission_denied_message to the PermissionRequiredMixin which can be configured with a message string that is passed to the PermissionDenied exception. By default, Django will then add the string representation of the PermissionDenied exception to the template context as exception.

If permission_denied_message is undefined, an empty string is passed to PermissionDenied.

Here are a couple of examples

1. Simple message.

If the current user does not have the app_label.view_secretmodel permission, they will be served the 403 page that contains the message "You do not have access to the request secret object.".

# models.py
from django.db import models
from django.contrib.auth.models import User

class SecretModel(models.Model):
    name = models.CharField(max_length=30)
    owner = models.ForeignKey(User, on_delete=models.CASCADE)

# views.py
from django.views.generic import DetailView
from guardian.mixins import PermissionRequiredMixin
from app_label.models import SecretModel

class SecretObjectDetail(PermissionRequiredMixin, DetailView):
    model = SecretModel
    permission_required = "app_label.view_secretmodel"
    raise_exception = True
    permission_denied_message = "You do not have access to the requested secret object."

<!-- 403.html -->
{% extends "base.html" %}
{% block content %}
<div class="container-fluid">
  <div class="alert alert-danger" role="alert">
    <h3 class="alert-heading">Permission Denied</h3>
    {{ exception|default:"You do not have permission to view this content." }}
  </div>
</div>
{% endblock content %}

2. More detailed message.

models.py and 403.html as above.

# views.py
from django.views.generic import DetailView
from guardian.mixins import PermissionRequiredMixin
from app_label.models import SecretModel

class SecretModelDetail(PermissionRequiredMixin, DetailView):
    model = SecretModel
    permission_required = "app_label.view_secretmodel"
    raise_exception = True

    def get_permission_denied_message(self):
        obj = self.get_object()
        return (
            f"You do not have permission to view the secret object '{obj.name}'. "
            f"Please ask the owner, {obj.owner.username}, to grant you permission."
        )

[Rainshaw]

And I think this function is a little complex.
In django.contrib.auth.mixins.PermissionRequiredMixin, we can find that it just return the self.permission_denied_message.

# in django/contrib/auth/mixins/PermissionRequiredMixin
    def get_permission_denied_message(self):
        """
        Override this method to override the permission_denied_message attribute.
        """
        return self.permission_denied_message

[Rainshaw]

We can make this be blank str '', and it would help with the get_permission_denied_message function to be simple.