AssertionError on accessing invalid codelist

Why are we doing this?

When codelists with invalid codelists are accessed, we get an assertion error, at a surprising location. The site 500s and the user goes to the 500 page then is redirected to the home page with the confusing message: "This codelist has not been published".

Sentry Issue: OPENCODELISTS-TD

Slack:

One of [the] old codelists is borked. ICD-10 coding system (presumably uploaded in old school yolomode) but is full of SNOMED CT codes. Someone or something tried to download it.

Offending URL: http://www.opencodelists.org/codelist/user/rriefu/icd/327e171a/ (best to access in a development environment with the production backup loaded).

Bots fairly regularly hit this page. We should avoid 500 errors which spam our Sentry issue list and lead to noisy alerts.

AssertionError: 
(2 additional frame(s) were not displayed)
...
  File "codelists/views/decorators.py", line 74, in wrapped_view
    rsp = view_fn(request, version, **view_kwargs)
  File "codelists/views/version.py", line 26, in version
    hierarchy = clv.codeset.hierarchy
  File "codelists/models.py", line 512, in codeset
    return self._old_style_codeset()
  File "codelists/models.py", line 522, in _old_style_codeset
    return Codeset.from_codes(set(self.codes), self.hierarchy)
  File "codelists/codeset.py", line 83, in from_codes
    assert sorted(codeset.codes()) == sorted(codes)

How will we know when it's done?

Accessing the page leads to a helpful error message and redirect and no unhandled error.

What are we doing?

Defining delivery tasks guidance

Start at codelists/views/version.py and follow the failing access through codelists/models.py and codelists/codeset.py, using the provided offending URL in a development environment with the production backup. Trace how invalid codelists reach the assertion and identify the existing error-handling path. Done means the page shows a helpful error, redirects appropriately, and produces no unhandled 500 error.