New handle_exception plugin hook, refs #1770

Also refs:
- https://github.com/simonw/datasette-sentry/issues/1
- https://github.com/simonw/datasette-show-errors/issues/2

datasette/forbidden.py

@@ -0,0 +1,20 @@
+from os import stat
+from datasette import hookimpl, Response
+
+
+@hookimpl(trylast=True)
+def forbidden(datasette, request, message):
+    async def inner():
+        return Response.html(
+            await datasette.render_template(
+                "error.html",
+                {
+                    "title": "Forbidden",
+                    "error": message,
+                },
+                request=request,
+            ),
+            status=403,
+        )
+
+    return inner