Cause npm run lint to fail if warnings are detected

[johnthagen]

Currently if npm run lint is run and warnings are detected, the process does not fail. This means that when npm run lint is run on CI, it will not fail if ESLint warnings are pushed.

Fix this by failing if any warnings are detected by ESLint.

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This change is 

[imnasnainaec]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnthagen)

[codecov-io]

Codecov Report

Merging #999 (27ccfb5) into master (43295a9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #999   +/-   ##
=======================================
  Coverage   49.72%   49.72%           
=======================================
  Files         236      236           
  Lines        6492     6492           
  Branches      393      393           
=======================================
  Hits         3228     3228           
  Misses       2974     2974           
  Partials      290      290           

Flag	Coverage Δ
backend	55.43% <ø> (ø)
frontend	43.53% <ø> (ø)

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