Add JSONified errors and a -panic option.

[patrickt]

Up until now, error handling has been done through panic(), which isn't so great, as it just dumps the error text and a bunch of goroutine-related detritus to stderr. This patch adds an error field to the toplevel returned object; if it's present, an error has been encountered.

There is a huge wart here—the error type returned from parser syntax errors is opaque, so we can't extract its position information. We'll just have to parse the information on the Haskell side. Blech.

[codecov-io]

Codecov Report

Merging #48 into master will decrease coverage by 1.42%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   60.57%   59.14%   -1.43%     
==========================================
  Files           1        1              
  Lines         662      678      +16     
==========================================
  Hits          401      401              
- Misses        229      245      +16     
  Partials       32       32

Impacted Files	Coverage Δ
goblin.go	59.14% <0%> (-1.43%)	⬇️

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