New dashboard: delete legacy HTML templates (part 3) (#12200)

Continue the work from:

* #12197

Author: humitos

readthedocs/proxito/exceptions.py

@@ -61,6 +61,10 @@ def __init__(self, domain, **kwargs):
         super().__init__(**kwargs)
 
 
+# All the following exceptions use the same template from ext-theme.
+# That template uses `not_found_subject` as a heading and copy of the page,
+# the rest of the page is exactly the same.
+# This can be tested by using `DEBUG = False` in El Proxito.
 class ProjectHttp404(ContextualizedHttp404):
     """
     Raised if a domain did not resolve to a project.
@@ -86,7 +90,7 @@ def __init__(self, domain, **kwargs):
 class SubprojectHttp404(ContextualizedHttp404):
     """Raised if a subproject was not found."""
 
-    template_name = "errors/proxito/404/no_subproject.html"
+    template_name = "errors/proxito/404/no_project.html"
     not_found_subject = pgettext_lazy("Names an object not found in a 404 error", "subproject")
 
     def __init__(self, project, **kwargs):
@@ -103,7 +107,7 @@ def __init__(self, project, **kwargs):
 class ProjectFilenameHttp404(ContextualizedHttp404):
     """Raised if a page inside an existing project was not found."""
 
-    template_name = "errors/proxito/404/no_project_page.html"
+    template_name = "errors/proxito/404/no_project.html"
     not_found_subject = pgettext_lazy(_not_found_subject_translation_context, "documentation page")
 
     def __init__(self, project, **kwargs):
@@ -125,7 +129,7 @@ class ProjectTranslationHttp404(ContextualizedHttp404):
     If a page isn't found, raise a ProjectPageHttp404.
     """
 
-    template_name = "errors/proxito/404/no_language.html"
+    template_name = "errors/proxito/404/no_project.html"
     not_found_subject = pgettext_lazy("Names an object not found in a 404 error", "translation")
 
     def __init__(self, project, **kwargs):
@@ -146,7 +150,7 @@ class ProjectVersionHttp404(ContextualizedHttp404):
     Note: The containing project can be a subproject.
     """
 
-    template_name = "errors/proxito/404/no_version.html"
+    template_name = "errors/proxito/404/no_project.html"
     not_found_subject = pgettext_lazy(
         _not_found_subject_translation_context, "documentation version"
     )