404 errors when attempting to download missing files in JI

[zenmonkeykstop]

Description

#5573 added a flash message to be displayed when a user attempts to download submissions that are missing from the application store. The message should be displayed on the original page - sources list or individual source page - but is being displayed on a 404 page for /col/None in the former case and /None in the latter.

Steps to Reproduce

• submit multiple files as a source in SI
• delete one from the file store on the app server
• attempt to download via the various singular and collective download options on the JI

Expected Behavior

Flash message is displayed, rest of behaviour as expected

Actual Behavior

404s as described.

error logs as follows:

[Tue Jan 19 15:34:43.815637 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:52968]   File "/var/www/securedrop/journalist_app/__init__.py", line 96, in _handle_http_exception
[Tue Jan 19 15:34:43.815640 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:52968]     handler = list(app.error_handler_spec['api'][error.code].values())[0]
[Tue Jan 19 15:34:43.815662 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:52968] KeyError
[Tue Jan 19 15:46:50.450869 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:53004] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found
[Tue Jan 19 15:46:51.078081 2021] [wsgi:error] [pid 9080:tid 129094201378560] [remote 127.0.0.1:53004] ERROR:flask.app:Found none when one was expected: No row was found for one()
[Tue Jan 19 15:48:00.505418 2021] [wsgi:error] [pid 9080:tid 129094201378560] [remote 127.0.0.1:53016] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found
[Tue Jan 19 15:48:01.232360 2021] [wsgi:error] [pid 9079:tid 129094200379136] [remote 127.0.0.1:53016] ERROR:flask.app:Found none when one was expected: No row was found for one()
[Tue Jan 19 15:48:15.683907 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:53028] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found
[Tue Jan 19 15:48:56.431962 2021] [wsgi:error] [pid 9080:tid 129094201378560] [remote 127.0.0.1:53032] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found
[Tue Jan 19 15:55:15.465256 2021] [wsgi:error] [pid 9080:tid 129094009444096] [remote 127.0.0.1:53048] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found
[Tue Jan 19 15:55:37.184542 2021] [wsgi:error] [pid 9079:tid 129093955651328] [remote 127.0.0.1:53058] ERROR:flask.app:File /var/lib/securedrop/store/D7NFN2FZRXJVQIU7FBLUCLPGMGM6YUEQ7WLNY6Y2U5IVFOGTQEFO6NGONBE6YKOBE54O6O4QR52VK7BPXBG26HXZXJPT33CCVBEJDSY=/1-homemade_flanker-msg.gpg not found

[sssoleileraaa]

I tested on nuc7 hardware, 1.7.0~rc2:

• When I click on the name of the file to try to download it (the /col path) I see the expected behavior.
• When I select the file and then click "Download", I hit the /None path and see the expected flash message but also the unexpected 404 Not Found message:

[sssoleileraaa]

• I also see an Internal Server Error on the /download_unread/None path (by clicking the X unread button)
• I was unable to repro in the dev environment