Issue #60: Improving error messages

[DavidHuebner]

• Added logic to deal with all three cases of errors (see description in Issue #60) especially now dealing with the case that raw_response.json() is not valid json and also printing status_code for all Exceptions now
• Added test cases for different error scenarios

[codecov-commenter]

Codecov Report

Merging #67 (b7b7829) into dev (318fa9c) will decrease coverage by 0.08%. The diff coverage is 92.52%.

@@            Coverage Diff             @@
##              dev      #67      +/-   ##
==========================================
- Coverage   90.12%   90.04%   -0.09%     
==========================================
  Files          15       15              
  Lines        1388     1437      +49     
==========================================
+ Hits         1251     1294      +43     
- Misses        137      143       +6     

Impacted Files	Coverage Δ
averbis/core/_rest_client.py	83.93% <89.61%> (-0.34%)	:arrow_down:
tests/test_client.py	100.00% <100.00%> (ø)
tests/test_project.py	100.00% <100.00%> (ø)

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[nadre]

Nice job @DavidHuebner! I skimmed over the changes and couldn't see any problems but I'll leave the merge to @reckart.