Fix colormap utils error suggestion code and add a test

[goanpeca]

Description

Type of change

• 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

How has this been tested?

• example: the test suite for my feature covers cases x, y, and z
• example: all tests pass with my change
• example: I check if my changes works with both PySide and PyQt backends
  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.

[codecov]

Codecov Report

Merging #5571 (0c03940) into main (151ca9c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5571   +/-   ##
=======================================
  Coverage   89.40%   89.41%           
=======================================
  Files         609      609           
  Lines       51170    51168    -2     
=======================================
  Hits        45750    45750           
+ Misses       5420     5418    -2     

Impacted Files	Coverage Δ
napari/utils/colormaps/colormap_utils.py	93.20% <ø> (ø)
napari/utils/colormaps/_tests/test_colormaps.py	100.00% <100.00%> (ø)
napari/components/_tests/test_layer_slicer.py	97.98% <0.00%> (-1.21%)	⬇️
napari/_qt/dialogs/qt_package_installer.py	81.81% <0.00%> (+0.39%)	⬆️
napari/utils/theme.py	94.04% <0.00%> (+0.59%)	⬆️
napari/utils/info.py	85.10% <0.00%> (+1.06%)	⬆️
napari/_qt/__init__.py	56.66% <0.00%> (+6.66%)	⬆️

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[Czaki]

I prefer to use match instead string assertions, but I'm ok with both.

[goanpeca]

I prefer to use match instead string assertions, but I'm ok with both.

TIL :) that is pretty cool, thanks for the suggestion!