MkDocs aborts serve if there are warnings for HTTP 404 errors

[prcr]

I was trying out MkDocs 1.2.2 and noticed the following change of behavior from 1.1.2:

When running mkdocs serve with strict mode, MkDocs stops on the first update to a page if there were any warnings for HTTP 404 errors:

$ mkdocs -V
mkdocs, version 1.2.2 from /home/prcr/.local/lib/python3.6/site-packages/mkdocs (Python 3.6)
$ mkdocs serve
INFO     -  Building documentation...
INFO     -  Cleaning site directory
INFO     -  Documentation built in 4.24 seconds
INFO     -  [11:48:31] Serving on http://127.0.0.1:8000/
INFO     -  [11:48:33] Browser connected: http://127.0.0.1:8000/special-thanks/
WARNING  -  [11:48:33] "GET /versions.json HTTP/1.1" code 404
INFO     -  [11:48:45] Detected file changes
INFO     -  Building documentation...
INFO     -  [11:48:50] Reloading browsers
INFO     -  [11:48:50] Browser connected: http://127.0.0.1:8000/special-thanks/
WARNING  -  [11:48:50] "GET /versions.json HTTP/1.1" code 404
INFO     -  [11:49:14] Detected file changes
INFO     -  Building documentation...

Aborted with 2 warnings in strict mode!

MkDocs 1.1.2 ignored the same warnings when also running in strict mode:

$ mkdocs -V
mkdocs, version 1.1.2 from /home/prcr/.local/lib/python3.6/site-packages/mkdocs (Python 3.6)
$ mkdocs serve
INFO    -  Building documentation... 
INFO    -  Cleaning site directory 
INFO    -  Documentation built in 5.50 seconds 
[I 210901 11:51:33 server:335] Serving on http://127.0.0.1:8000
INFO    -  Serving on http://127.0.0.1:8000
[I 210901 11:51:33 handlers:62] Start watching changes
INFO    -  Start watching changes
[W 210901 11:51:34 web:2243] 404 GET /versions.json (127.0.0.1) 6.18ms
WARNING -  404 GET /versions.json (127.0.0.1) 6.18ms
[I 210901 11:51:34 handlers:135] Browser Connected: http://127.0.0.1:8000/
INFO    -  Browser Connected: http://127.0.0.1:8000/
[I 210901 11:51:40 watcher:111] Running task: builder (delay: None)
INFO    -  Running task: builder (delay: None)
INFO    -  Building documentation... 
[I 210901 11:51:45 handlers:95] Reload 1 waiters: /home/prcr/Codacy/git/docs/submodules/chart/docs/maintenance/uninstall.md
INFO    -  Reload 1 waiters: /home/prcr/Codacy/git/docs/submodules/chart/docs/maintenance/uninstall.md
[I 210901 11:51:46 handlers:82] Ignore: /home/prcr/Codacy/git/docs/docs/related-tools/local-analysis/images/run-analysis-through-build-server.png
INFO    -  Ignore: /home/prcr/Codacy/git/docs/docs/related-tools/local-analysis/images/run-analysis-through-build-server.png
[W 210901 11:51:46 web:2243] 404 GET /versions.json (127.0.0.1) 5.92ms
WARNING -  404 GET /versions.json (127.0.0.1) 5.92ms
[I 210901 11:51:46 handlers:135] Browser Connected: http://127.0.0.1:8000/
INFO    -  Browser Connected: http://127.0.0.1:8000/

I wonder if it was a conscious decision to change this behavior in MkDocs 1.2 or if we could go back to the previous behavior of (safely) ignoring the HTTP 404 errors when serving the pages.

[oprypin]

Worked out a fix in #2607