fix: handle InternalServerErrors (ClientError subclass)

[pjenvey]

by treating ClientError as we originally treated boto2's
JSONResponseError

centralize endpoint's ClientError handling into _boto_err (kills
_overload_err as it's all now handled in one place)

Issue #1138

[codecov-io]

Codecov Report

Merging #1140 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1140      +/-   ##
==========================================
+ Coverage   99.76%   99.76%   +<.01%     
==========================================
  Files          60       60              
  Lines        9965     9966       +1     
==========================================
+ Hits         9942     9943       +1     
  Misses         23       23

Impacted Files	Coverage Δ
autopush/websocket.py	100% <ø> (ø)	⬆️
autopush/tests/test_web_base.py	100% <100%> (ø)	⬆️
autopush/web/base.py	100% <100%> (ø)	⬆️
autopush/tests/test_websocket.py	100% <100%> (ø)	⬆️

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