Ignore global error variables (implements #2)

[leonklingele]

Error variables are variables starting with 'err' or 'Err' which
were made by either errors.New or fmt.Errorf.

There still is leeway for improvement such as checking if the
node implements the error interface. This would allow us to
whitelist custom error variables.

Fixes #2

[codecov-io]

Codecov Report

Merging #3 into master will increase coverage by 4.46%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   54.68%   59.15%   +4.46%     
==========================================
  Files           2        2              
  Lines          64       71       +7     
==========================================
+ Hits           35       42       +7     
  Misses         27       27              
  Partials        2        2

Impacted Files	Coverage Δ
check_no_globals.go	91.3% <100%> (+1.56%)	⬆️

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[leighmcculloch]

@leonklingele Thanks for submitting this so promptly! Thoughts below, let me know what you think.

[leonklingele]

Updated to also catch customly-typed error variables, "fake errors" will no longer be caught.
PTAL @leighmcculloch :)

[leighmcculloch]

This is so exciting.

[leonklingele]

This should be fine now @leighmcculloch :)

[leighmcculloch]

@leonklingele 👍, is 3f4f961 better?

[leonklingele]

lgtm! :) Thanks for the improvements.