Use sphinx-notfound-page by default to create a generic 404 page

We use the sphinx-notfound-page extension to create a generic page for
each project. By default we use the same Maze Found ASCII art, but the
user can customize it by defining specific configs from the sphinx-notfound-page.

readthedocs/core/views/__init__.py

@@ -132,7 +132,7 @@ def server_error_404(request, exception=None, template_name='404.html'):  # pyli
             return response
 
     # Try to serve custom 404 pages if it's a subdomain/cname
-    if request.subdomain or request.cname:
+    if getattr(request, 'subdomain', False) or getattr(request, 'cname', False):
         return server_error_404_subdomain(request, template_name)
 
     # Return the default 404 page generated by Read the Docs

readthedocs/doc_builder/python_environments.py

@@ -307,6 +307,7 @@ def install_core_requirements(self):
                 ),
                 'sphinx-rtd-theme<0.5',
                 'readthedocs-sphinx-ext<0.6',
+                'sphinx-notfound-page',
             ])
 
         cmd = copy.copy(pip_install_cmd)
@@ -429,7 +430,10 @@ def install_core_requirements(self):
         if self.config.doctype == 'mkdocs':
             pip_requirements.append('mkdocs')
         else:
-            pip_requirements.append('readthedocs-sphinx-ext')
+            pip_requirements.extend([
+                'readthedocs-sphinx-ext',
+                'sphinx-notfound-page',
+            ])
             requirements.extend(['sphinx', 'sphinx_rtd_theme'])
 
         cmd = [
@@ -443,7 +447,7 @@ def install_core_requirements(self):
         cmd.extend(requirements)
         self.build_env.run(
             *cmd,
-            cwd=self.checkout_path  # noqa - no comma here in py27 :/
+            cwd=self.checkout_path,
         )
 
         pip_cmd = [

readthedocs/doc_builder/templates/doc_builder/conf.py.tmpl

@@ -140,3 +140,16 @@ if 'extensions' in globals():
     extensions.insert(0, "readthedocs_ext.readthedocs")
 else:
     extensions = ["readthedocs_ext.readthedocs"]
+
+
+# Define default 404 page body
+if 'notfound_context' not in globals():
+    notfound_context = {
+        'body': '''
+<h1>Page not found</h1>
+
+<p>Sorry, we couldn't find that page.</p>
+
+<p>Try using the search box or go to the <a href="/">homepage</a>.</p>
+''',
+    }

readthedocs/rtd_tests/tests/test_doc_building.py

@@ -1229,6 +1229,7 @@ def test_install_core_requirements_sphinx(self, checkout_path):
             'sphinx',
             'sphinx-rtd-theme',
             'readthedocs-sphinx-ext',
+            'sphinx-notfound-page',
         ]
         requirements = self.base_requirements + requirements_sphinx
         args = self.pip_install_args + requirements
@@ -1356,6 +1357,7 @@ def test_install_core_requirements_sphinx_conda(self, checkout_path):
         pip_requirements = [
             'recommonmark',
             'readthedocs-sphinx-ext',
+            'sphinx-notfound-page',
         ]
 
         args_pip = [