[IMP] add information in error message

[mart-e]

Not always clear what was the issue with only the keyword.

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

After writing a long article, I got this warning message but I had to look in the code to understand what was wrong (used {content} while I wanted {filename}). Not really clear what we meant by "Replacement Indicator".

Before this commit:

$ make devserver
…
Serving site at: http://127.0.0.1:8000 - Tap CTRL-C to stop
[14:46:30] WARNING  Replacement Indicator 'content' not recognized, skipping replacement                                                                                                           contents.py:360
Done: Processed 210 articles, 24 drafts, 0 hidden articles, 1 page, 1 hidden page and 0 draft pages in 3.53 seconds.

After this commit

$ make devserver
…
Serving site at: http://127.0.0.1:8000 - Tap CTRL-C to stop
[14:45:47] WARNING  Replacement Indicator 'content' not recognized in '{content}/2024/my-shinny-article.md', skipping replacement                                           contents.py:360
Done: Processed 210 articles, 24 drafts, 0 hidden articles, 1 page, 1 hidden page and 0 draft pages in 3.05 seconds.

An alternative patch could be simply:

- "Replacement Indicator '%s' not recognized, skipping replacement"
+ "Replacement Indicator '{%s}' not recognized, skipping replacement"

I guess I would have understood as well. Let me know what you prefer.