Use Perish() on parse errors.

[patrickt]

Also ensures that erroneous output goes to stderr.

[codecov-io]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   59.14%   59.14%           
=======================================
  Files           1        1           
  Lines         678      678           
=======================================
  Hits          401      401           
  Misses        245      245           
  Partials       32       32

Impacted Files	Coverage Δ
goblin.go	59.14% <0%> (ø)	⬆️

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