Unhelpful Theme Errors

[joegm]

When writing and experimenting with themes I've noticed that if there is anything wrong with a theme (syntax error, missing key, misspelled key) and you try to select it with :theme <mytheme> the error is always:

Invalid theme: 'ui.selection' required

This message shows up even when ui.selection is present and is most of the time completely irrelevant to the actual issue with the theme. I like the theming in Helix but this makes it a lot harder to work with. I'd prefer individual errors for, say:

• Syntax errors like missing commas, with line number.
• Non-existent/misspelled keys.
• Required keys that aren't implemented, specifying which key it is.

[0rphee]

For the time being I found this site useful for finding typos and errors in .tomlfiles for themes

[kirawi]

Partially addressed by cargo xtask themelint introduced in #3234