Improve error message when representing incompatible value for Union

[kitchoi]

If I try to create a trait type like this:

Union(Dict(), "none")

I get an error message:

Union trait declaration expects a trait type or an instance of trait type or None, but got none instead

This PR improves the error message so that it reads:

Union trait declaration expects a trait type or an instance of trait type or None, but got 'none' instead

which is much clearer about what went wrong.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[codecov-commenter]

Codecov Report

Merging #1103 into master will decrease coverage by 1.94%.
The diff coverage is 68.31%.

@@            Coverage Diff             @@
##           master    #1103      +/-   ##
==========================================
- Coverage   76.15%   74.21%   -1.95%     
==========================================
  Files          54       78      +24     
  Lines        6493     8469    +1976     
  Branches     1263     1610     +347     
==========================================
+ Hits         4945     6285    +1340     
- Misses       1205     1806     +601     
- Partials      343      378      +35     

Impacted Files	Coverage Δ
traits/api.py	100.00% <ø> (+9.67%)	⬆️
traits/base_trait_handler.py	61.76% <ø> (ø)
traits/observers/_i_notifier.py	0.00% <0.00%> (ø)
traits/observers/events.py	0.00% <0.00%> (ø)
traits/traits.py	75.10% <ø> (-2.45%)	⬇️
traits/util/resource.py	15.25% <ø> (ø)
traits/observers/_generated_parser.py	51.96% <51.96%> (ø)
traits/trait_types.py	72.29% <80.00%> (-0.16%)	⬇️
traits/observers/parsing.py	95.34% <95.34%> (ø)
traits/observers/_trait_event_notifier.py	96.61% <96.61%> (ø)
... and 57 more

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[mdickinson]

LGTM