TailCurve argument validation

[genedan]

Addresses #413.

[codecov-commenter]

Codecov Report

Base: 83.92% // Head: 83.93% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (8eb21ec) compared to base (083a2bb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
+ Coverage   83.92%   83.93%   +0.01%     
==========================================
  Files          50       50              
  Lines        4448     4452       +4     
  Branches      729      731       +2     
==========================================
+ Hits         3733     3737       +4     
  Misses        513      513              
  Partials      202      202              

Flag	Coverage Δ
unittests	83.93% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
chainladder/tails/curve.py	60.41% <100.00%> (+1.72%)	⬆️

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[jbogaardt]

Thank you @genedan. No doubt this was needed. Error handling in chainladder-python has received limited focus, so I expect similar low hanging fruit exists elsewhere too. Thanks for the PR!