Throw errors when hapi integration fails

[jonpacker]

At the moment the hapi integration fails by sending null as the error object, which results in this cryptic an unhelpful error from hapi:

[start] Debug: internal, implementation, error
    Error: Cannot throw non-error object
    at module.exports.internals.Manager.execute (.../node_modules/hapi/lib/toolkit.js:44:33)
    at process._tickCallback (internal/process/next_tick.js:68:7)

I suspect the most often case for this happening is ours, when an HS256-signed JWT is being sent. This pull request adds a couple of error messages so that the user gets some information about what is actually going wrong. I kind of guessed about what they say, my domain knowledge here is not great. Feel free to make them more informative.