Refactor error handling.

[James-Burgess]

• Split nests into functions
• Update fallback error to include api response.
• Assert all branches end in default error

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.56757% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 92.96%. Comparing base (f12eecb) to head (6d9dbeb).
Report is 1 commits behind head on master.

Files	Patch %	Lines
herepy/routing_api.py	67.56%	12 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   93.25%   92.96%   -0.29%     
==========================================
  Files          26       26              
  Lines        1735     1749      +14     
==========================================
+ Hits         1618     1626       +8     
- Misses        117      123       +6     

Flag	Coverage Δ
unittests	92.96% <67.56%> (-0.29%)	⬇️

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[abdullahselek]

Awesome 😎 thank you!