ERR: Better error msg when merging on tz-aware and tz-naive columns

[reidy-p]

• closes ERR: need a better error message when trying to compare incompat types on merge #15800
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19525 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19525      +/-   ##
==========================================
- Coverage   91.62%   91.62%   -0.01%     
==========================================
  Files         150      150              
  Lines       48680    48681       +1     
==========================================
- Hits        44603    44602       -1     
- Misses       4077     4079       +2

Flag	Coverage Δ
#multiple	89.98% <100%> (-0.01%)	⬇️
#single	41.73% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/reshape/merge.py	94.22% <100%> (ø)	⬆️
pandas/util/testing.py	83.64% <0%> (-0.21%)	⬇️

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[TomAugspurger]

Thanks!