Make ErrorHandlingEmitterToken optional, default to calling window.onError

[ganemone]

Fixes #59

[codecov]

Codecov Report

Merging #60 into master will increase coverage by 6.88%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master   #60      +/-   ##
=======================================
+ Coverage   71.11%   78%   +6.88%     
=======================================
  Files           2     2              
  Lines          45    50       +5     
  Branches       15    16       +1     
=======================================
+ Hits           32    39       +7     
+ Misses         11     9       -2     
  Partials        2     2

Impacted Files	Coverage Δ
src/client.js	81.48% <85.71%> (+13.29%)	⬆️

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