Print a better error message when an assertion fails

[DilumAluthge]

[codecov-commenter]

Codecov Report

Merging #413 (f845ff2) into master (c41467b) will decrease coverage by 0.16%. The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
- Coverage   95.45%   95.28%   -0.17%     
==========================================
  Files          36       36              
  Lines        1760     1762       +2     
==========================================
- Hits         1680     1679       -1     
- Misses         80       83       +3     

Impacted Files	Coverage Δ
src/types/timezone.jl	89.18% <25.00%> (-7.96%)	:arrow_down:

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[DilumAluthge]

I don't think the original code exists anymore, so I think this PR is now stale.