python: send html error page on Packages errors

Like the C bridge send a proper formatted `fail.html` back to the
client. This does not handle anything any other then 404's.

Co-authored-by: Jelle van der Waa <jvanderwaa@redhat.com>

pyproject.toml

@@ -14,9 +14,13 @@ where = ["src"]
 include = [
   "cockpit",
   "cockpit.channels",
+  "cockpit.data",
   "systemd_ctypes",
 ]
 
+[tool.setuptools.package-data]
+"cockpit.data" = ["*.html"]
+
 [tool.mypy]
 mypy_path = 'src'
 

src/cockpit/channels/packages.py

@@ -48,8 +48,11 @@ class PackagesChannel(Channel):
         self.send_message(status=200, reason='OK', headers={k: v for k, v in headers.items() if v is not None})
 
     def http_error(self, status, message):
-        self.send_message(status=status, reason='ERROR')
-        self.send_data(message.encode('utf-8'))
+        # with (importlib.resources.file('cockpit.data') / 'data' / 'fail.html').open() ...  (from py3.7)
+        fail_path = __file__.removesuffix('/channels/packages.py') + '/data/fail.html'
+        template = __loader__.get_data(fail_path)
+        self.send_message(status=status, reason='ERROR', headers={'Content-Type': 'text/html; charset=utf-8'})
+        self.send_data(template.replace(b'@@message@@', message.encode('utf-8')))
 
     def do_done(self):
         assert not self.post

src/cockpit/data/fail.html

@@ -0,0 +1 @@
+../../../src/common/fail.html