Improve error message when OpenAPI document fails validation

[dbanty]

closes #129

[codecov]

Codecov Report

Merging #131 into main will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #131   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           40        40           
  Lines         1139      1149   +10     
=========================================
+ Hits          1139      1149   +10     

Impacted Files	Coverage Δ
openapi_python_client/__init__.py	100.00% <100.00%> (ø)
openapi_python_client/parser/openapi.py	100.00% <100.00%> (ø)

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