Use the right type for the ValidationError constructor

[NGTOne]

The current ValidationError constructor definition is inherited from the base Error, which gives it a signature of constructor(message?: string, fileName?: string, lineNumber?: string). This is at odds with the actual definition and use of ValidationError, which means TypeScript consumers of this library that wish to use this error themselves must currently work around the issue like so:

throw new ValidationError({body: errors} as any);

[codecov]

Codecov Report

Merging #68 (a29980b) into master (4c162aa) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #68   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           26        26           
=========================================
  Hits            26        26           

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[simonplend]

Thank you so much for this pull request, I really appreciate it.

I'll be including this fix in the next release, which I hope to get out within the next couple of weeks.