Use definite article for single file error case; improve functional test output

[eloquence]

Status

Ready for review

Description of Changes

• Distinguishes single/multiple file error case. See @deeplow's comment in https://weblate.securedrop.org/translate/securedrop/securedrop/en/?checksum=aec87110942e8803#comments
• Registers pytest assertion rewriting for our helper functions so test failures show helpful string comparison when an assertion fails.

Testing

Error message behavior

On this PR's branch:

1. Spin up Docker env
2. Remove all files from disk in the container's /var/lib/securedrop/store directory
3. Attempt to download single file by clicking its filename directly in the Journalist Interface
4. • Observe that single file error message is displayed
5. Attempt to download single file via the checkbox interface
6. • Observe that single file error message is displayed
7. Attempt to download multiple files
8. • Observe that multiple file error message is displayed

Improved assertion output

1. Break one of the tests like so:

diff --git a/securedrop/tests/functional/journalist_navigation_steps.py b/securedrop/tests/functional/journalist_navigation_steps.py
index 3f3813a7d..da8e6d825 100644
--- a/securedrop/tests/functional/journalist_navigation_steps.py
+++ b/securedrop/tests/functional/journalist_navigation_steps.py
@@ -1189,7 +1189,7 @@ class JournalistNavigationStepsMixin:
         if single_file:
             error_msg = (
                 "Your download failed because the file could not be found. An admin can find "
-                + "more information in the system and monitoring logs."
+                + "more information in the system and monitoring logs. XYZZY"
             )
         else:
             error_msg = (

2. Run it via securedrop/bin/dev-shell bin/run-test -s -v tests/functional/test_journalist.py -k test_download_source_unread^
3. • Observe that the assertion output includes a diff, unlike the output in previous test failures

[eloquence]

(CI's probably going to fail, will let it do its thing and then update affected tests)

[codecov-commenter]

Codecov Report

Merging #5905 (e8e1df8) into develop (fedce7a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5905   +/-   ##
========================================
  Coverage    85.49%   85.49%           
========================================
  Files           53       53           
  Lines         3902     3902           
  Branches       484      484           
========================================
  Hits          3336     3336           
  Misses         455      455           
  Partials       111      111           

Impacted Files	Coverage Δ
securedrop/journalist_app/col.py	82.19% <ø> (ø)

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[rmol]

If we wanted to show the single file error message in the case where one file is selected and "Download Selected" is clicked, we could check the length of the list of things to delete here.

I'm approving as is, and will merge unless you want to try that. The test plan went fine:

1. Spin up Docker env
2. Remove all files from disk in the container's /var/lib/securedrop/store directory
3. Attempt to download single file
4. • Observe that single file error message is displayed
5. Attempt to download multiple files
6. • Observe that multiple file error message is displayed

[eloquence]

That would be nicer, I'll take another look, thanks @rmol

[deeplow]

Wow. It is super interesting how little one realizes as a translator the engineering effort & implications of what looked like a simple thing.

Thank you for addressing this so quickly and keep up the good work!!

[eloquence]

Thanks for the suggestion @rmol. This is now done and the correct message should be shown regardless of download method.

As part of looking at functional test output, I got frustrated that the assert failures didn't include diffs, and added assert rewrite logic which should fix this at least for the functional test helpers.

staging-test-* and translation-tests have been balking at me, but I'm not sure yet if this is related to anything in this PR.

[eloquence]

Looks like it was just flaky tests, as usual :/

[rmol]

Looks good. Tested single/multiple downloads, and verified the pytest assert rewriting in support functions, which is a nice enhancement.