Feature: improve request error structure

[yanyongyu]

RequestTimeout and RequestFailed are subclass of RequestError now.

[codecov]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 17.82%. Comparing base (3865ae3) to head (d7ca09e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
githubkit/core.py	0.00%	4 Missing ⚠️
githubkit/exception.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   17.82%   17.82%           
=======================================
  Files        4842     4842           
  Lines      223455   223460    +5     
=======================================
+ Hits        39835    39838    +3     
- Misses     183620   183622    +2     

Flag	Coverage Δ
unittests	17.82% <53.33%> (+<0.01%)	⬆️

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