Invalid JWT token results in error object in callback as string rather than object

[JayCarney]

Not sure if this extends to other error cases, but when a malformed Bearer token is sent to .validate(token, callback), the error object passed to the callback is a string value of Not a valid JWT token

expected behaviour:

• Invalid JWT token values result in an error object with keys name and message like other error objects

[BriantOliveira]

I'm currently having the same issue, I verified all my info and it's correct, Cognito region, pool id, and token user, but I'm still getting this issue.

[ss-vijayvavdiya]

I have handled the error incase invalid jwt and made PR

[ss-vijayvavdiya]

@JayCarney
PR #70 is now merged. Can you please verify the issue and close it?