Adding util tests to increase coverage
Adding errors.test.js

This error-centric test file is intended to serve as executable
documentation for the error codes returned by the validator. Given
the nature of validation, errors are extremly likely to occur and
clients will want to understand the error structure and what
information is provided back in them.
More error tests
Adding required and unreachable tests
Simplifying error tests

Got rid of errorTest helper function, which resulted in less typing
but I think less readability. Reorganized to group by method, then
error type.
Add tests for method delegation to errors.test.js