Add built-in monitor for critical errors

[codecov[bot]]

Codecov Report

Merging #329 (59347b3) into master (b40c322) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   73.92%   73.97%   +0.05%     
==========================================
  Files          68       68              
  Lines        2991     2997       +6     
  Branches      451      452       +1     
==========================================
+ Hits         2211     2217       +6     
  Misses        718      718              
  Partials       62       62              

Impacted Files	Coverage Δ
spidermon/contrib/scrapy/monitors.py	98.22% <100.00%> (+0.06%)	:arrow_up:

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[rennerocha]

Closes #303