Add function that generates error message

[h4apo]

Add function generateErrorMessage that recursively goes through the validation error and generates a message containing the reason of failed validation.

[codecov-io]

Codecov Report

Merging #167 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #167   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          429       429           
  Branches        61        61           
=========================================
  Hits           429       429           

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[gphfour]

@h4apo I suggest we don't merge the PR since the ValidationError message is mostly meant for technical people and it shouldn't be shown to users in normal circumstances. Additionally, the users should be shown a more user friendly message instead of a detail description of the error if that's the case.