Support custom exception handler

[An4ik]

I have implemented custom exception handler function (docs) and found that authentication views return (obtain, refresh) the data in different format. So, that's why I suggested the solution by this commit.

[codecov]

Codecov Report

Merging #424 into master will decrease coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
- Coverage   90.67%   90.34%   -0.34%     
==========================================
  Files          14       12       -2     
  Lines         847      818      -29     
  Branches       29       29              
==========================================
- Hits          768      739      -29     
  Misses         66       66              
  Partials       13       13

Flag	Coverage Δ
#codecov	90.34% <100%> (-0.34%)	⬇️
#dj110	87.04% <100%> (-0.45%)	⬇️
#dj111	87.04% <100%> (-0.45%)	⬇️
#dj18	89.48% <100%> (-0.36%)	⬇️
#dj19	89.48% <100%> (-0.36%)	⬇️
#drf31	89.48% <100%> (-0.36%)	⬇️
#drf32	89.48% <100%> (-0.36%)	⬇️
#drf33	89.48% <100%> (-0.36%)	⬇️
#drf34	90.34% <100%> (-0.34%)	⬇️
#drf35	89.97% <100%> (-0.35%)	⬇️
#drf36	89.97% <100%> (-0.35%)	⬇️
#py27	90.34% <100%> (-0.34%)	⬇️
#py33	89.11% <100%> (-0.38%)	⬇️
#py34	89.97% <100%> (+0.48%)	⬆️
#py35	87.04% <100%> (?)
#py36	87.04% <100%> (?)

Impacted Files	Coverage Δ
rest_framework_jwt/settings.py	100% <ø> (ø)	⬆️
rest_framework_jwt/views.py	92.5% <100%> (ø)	⬆️
rest_framework_jwt/models.py
rest_framework_jwt/utils.py

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