Add information about syntax_style value in error message for theme…

… validation (#5602)

# Description
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The current message do not provide information about the used value that
is important to debug a problem.

## Type of change
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- [x] Bug-fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

# References
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brisvag/napari-gruvbox#1

# How has this been tested?
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      as there are small differences between the two Qt bindings.  

## Final checklist:
- [ ] My PR is the minimum possible work for the desired functionality
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] If I included new strings, I have used `trans.` to make them
localizable.
For more information see our [translations
guide](https://napari.org/developers/translations.html).

napari/utils/theme.py

@@ -86,9 +86,10 @@ def _ensure_syntax_style(value: str) -> str:
         from pygments.styles import STYLE_MAP
 
         assert value in STYLE_MAP, trans._(
-            "Incorrect `syntax_style` value provided. Please use one of the following: {syntax_style}",
+            "Incorrect `syntax_style` value: {value} provided. Please use one of the following: {syntax_style}",
             deferred=True,
             syntax_style=f" {', '.join(STYLE_MAP)}",
+            value=value,
         )
         return value