instanceof fails for ValidationError when there are multiple toi versions in one monorepo

[gphfour]

The instanceof operator doesn't always work when there are multiple versions of toi in one monorepo, for example. Different require statements result in multiple ValidationError classes and the check that is performed with instanceof is no longer reliable.

One approach on how can this be fixed is by introducing a boolen flag, like isToi, to the ValidationError class that will identify the error. The Joi library has the same approach.