add a custom error handler to key-auth middleware

[hyacinthus]

link to #1846

[codecov]

Codecov Report

Merging #1847 (7543e88) into master (bb7f222) will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1847      +/-   ##
==========================================
+ Coverage   89.57%   90.18%   +0.61%     
==========================================
  Files          32       32              
  Lines        2686     2690       +4     
==========================================
+ Hits         2406     2426      +20     
+ Misses        180      169      -11     
+ Partials      100       95       -5     

Impacted Files	Coverage Δ
middleware/key_auth.go	96.55% <100.00%> (+29.88%)	⬆️

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[hyacinthus]

I'll update doc after pr accepted.

And it's hard to add a test to this patch...