Improve errors returned from httpClientHandler.

[dlongley]

Addresses #19.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (22410fd) to head (112116f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #20   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           55        83   +28     
=========================================
+ Hits            55        83   +28     

Files with missing lines	Coverage Δ
lib/index.js	100.00% <100.00%> (ø)

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