fix: endpoint now returns a JSON-based 404 for all 404's

[bbangert]

Closes #756

[codecov-io]

Codecov Report

Merging #1216 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1216   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          60      60           
  Lines       10191   10210   +19     
======================================
+ Hits        10191   10210   +19

Impacted Files	Coverage Δ
autopush/web/base.py	100% <100%> (ø)	⬆️
autopush/tests/test_integration.py	100% <100%> (ø)	⬆️
autopush/http.py	100% <100%> (ø)	⬆️

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