Notes from Estimation:

• Verify that this is not already being handled by other content handlers. There are ~4 other handlers that make various checks, so make sure that it's not already covered by top level error handling.
  • More specific checks allow for more detailed/specific error messages and telemetry.
• Add logging for the error (if it is not already handled)
  • Relates to https://phabricator.wikimedia.org/T421887 for specific 404 error types. We can add these differentiations later; this ticket should only focus on core error handling for now.

Change #1270438 had a related patch set uploaded (by Atieno; author: Atieno):

[mediawiki/extensions/WikimediaCustomizations@master] Attribution: Add handler tests

https://gerrit.wikimedia.org/r/1270438

Update on this being a blocker to launch::

The Attribution endpoint returns 404 for a missing page for example Missing page handled by the call to checkHasContent. The Asserts that happen earlier in the codebase are not fatal in Production(by default), only in development/test/debug mode so the local throws I had are a false alarm...However if in the future assertion failures were magically enabled in prod then we would have these crashes, but for now the "we don't raise 403 and 404" is technically not a blocker.

Change #1270438 merged by jenkins-bot:

[mediawiki/extensions/WikimediaCustomizations@master] Attribution: Add handler tests and 403, 404 logging

https://gerrit.wikimedia.org/r/1270438