Improve treatment of the "Template Part Not Found" message

[kjellr]

When there's an error with a theme's template part, the site displays a message like this:

This is helpful for development, but it's also a very confusing thing to show to users. Can we adjust this so that it fails more gracefully? (In my testing, this message appears whether or not WP_DEBUG is on or off)

[talldan]

@kjellr I made an issue related to this too - #36510, it might be possible to close one of them as a duplicate.

The message doesn't seem to work when I test it, but maybe that depends on what kind of template part it is (file vs post). It looks like I'm testing the latter and you're testing the former.

[talldan]

BTW, my instinct was that the block could show its placeholder instead of the message so that a user can at least pick a different template part, but there might be other options.

[kjellr]

Hmm. These seem similar, but might be slightly different? In the case here, I'm mostly concerned about these messages showing up to front-end visitors.

[talldan]

Yes, I didn't quite catch that this was on the front-end of the site. 👍

Do you think this should be fixed for 5.9? My feeling is that it probably should be, so I'll add it to the project board.

[kjellr]

Yeah, I think it should be. Thank you!

[Mamaduka]

@kjellr, PR is ready for the review :)