Add a gracefull method to set error Context#GraceError.

[sleagon]

Add a gracefull method to add error to context#Error #2017

[sleagon]

@thinkerou can you take a look?

[codecov[bot]]

Codecov Report

Merging #2018 into master will decrease coverage by 0.17%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2018      +/-   ##
==========================================
- Coverage   98.49%   98.31%   -0.18%     
==========================================
  Files          40       40              
  Lines        2257     2261       +4     
==========================================
  Hits         2223     2223              
- Misses         18       22       +4     
  Partials       16       16

Impacted Files	Coverage Δ
context.go	97.79% <0%> (-0.88%)	:arrow_down:

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[sleagon]

feel free to close this, by the way, if anyone want to catch error and format the response especially doing I18N at the same time, here is a extremely simple and clean package to do this.