Add custom service error type support

[mdurling]

Minor change to allow custom error types so that system isn't locked into the predefined set.
This is a totally backwards compatible change. The following code segments are equivalent:

if (isServiceError(error)) {
  if (error.type === 'NotFoundError') {
    return res.status(404).send({ error })
  }
}

if (isServiceError(error, 'NotFoundError')) {
    return res.status(404).send({ error })
  }
}

Services can now return a custom error:

return ServiceError({ type: 'CustomError', message: 'Custom error message' })

Which can be handled using the new signature:

if (isServiceError(error, 'CustomError')) {
    return res.status(400).send({ error })
  }
}

[levi217]

Nice! It might make sense to define a base service error type to express response middleware to do some out of the box handling for well known situations like 404, 500, 401, 400, etc. Otherwise we'll see various apis implement subsets of this. Even custom scenarios could extend on this. Just a thought.

[avatarneil]

I would love for this to inherit from mds-utils/exceptions.ts if possible at some point