Exit nonzero code if codecov fails to upload

[dsherry]

@ctduffy just hit a timeout on her PR, so I figure we should clean this one up.

From the codecov binary help output:

  --required            If Codecov fails it will exit 1 - possibly failing the
                        CI build.

[codecov]

Codecov Report

Merging #810 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #810   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files         150      150           
  Lines        5781     5781           
=======================================
  Hits         5753     5753           
  Misses         28       28           

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[angela97lin]

Nice, LGTM!