helpful color clash error message

[Mattriks]

• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• Throws a helpful error message if there is a clash of color scales. Works for the examples in
  Color facetting fails when combined with a contour layer #1098, Exception in apply_scales() when using multiple layers+DataFrame+color keys #1171, Layering two sound plots causes an unexpected error #1421
• closes Layering two sound plots causes an unexpected error #1421

[codecov-commenter]

Codecov Report

Merging #1448 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1448   +/-   ##
=======================================
  Coverage   89.40%   89.40%           
=======================================
  Files          39       39           
  Lines        4379     4380    +1     
=======================================
+ Hits         3915     3916    +1     
  Misses        464      464           

Impacted Files	Coverage Δ
src/scale.jl	97.60% <100.00%> (+<0.01%)	⬆️

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