Expand error handler interfaces

[izumin5210]

WHY

In the next version, i plan to make handlers receive request/response values...

WHAT

Make interfaces extendable

[codecov-io]

Codecov Report

Merging #11 into master will decrease coverage by 26.04%.
The diff coverage is 48.52%.

@@             Coverage Diff             @@
##           master      #11       +/-   ##
===========================================
- Coverage      84%   57.95%   -26.05%     
===========================================
  Files           3        3               
  Lines          50       88       +38     
===========================================
+ Hits           42       51        +9     
- Misses          7       36       +29     
  Partials        1        1

Impacted Files	Coverage Δ
interceptors.go	24% <15%> (-30.55%)	⬇️
utils.go	40.9% <40.9%> (-34.1%)	⬇️
handlers.go	87.8% <80.76%> (-8.97%)	⬇️

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[izumin5210]

coverage is down because increase loc for streaming server