Add raise_as context manager to convert ValidationError

[mcflugen]

I've added a new function, raise_as, that converts requireit.ValidationError to some other error.

I thought this could be useful for libraries that use requireit for validation, but want to expose their own exception types rather than requireit.ValidationError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ffb14c6) to head (51c5dbf).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #40   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          448       460   +12     
=========================================
+ Hits           448       460   +12     

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