Hardhat: call process.exit on SIGINT

[cgewecke]

#606

[codecov-io]

Codecov Report

Merging #616 (50a58e9) into master (5062c37) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #616   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          22       22           
  Lines         983      984    +1     
=======================================
+ Hits          970      971    +1     
  Misses         13       13           

Impacted Files	Coverage Δ
plugins/hardhat.plugin.js	93.18% <100.00%> (ø)
plugins/resources/nomiclabs.utils.js	100.00% <100.00%> (ø)

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[cgewecke]

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