Unify error format

[dompuiu]

Expected Behaviour

Let's say we have the following code:

auth(config)
  .then(token => console.log(token))
  .catch(error => console.log(`Error retrieving access token. ${error}`));

Sometimes jwt-auth throws a native error (eg. if we feed a public key as a private key). Other times it throws a native object (for JSON errors thrown by the API). It will be helpful to have one type of error.

Actual Behaviour

In the case when an object is thrown as an error, the native object toString method will return
[object Object]. Our error looks like Error retrieving the access token. [object Object] which is not optimal.

[shazron]

Can we wrap this into #9?

[dompuiu]

Yes. But it is @Aaronius fault that he doesn't read the comments from the PR request form. :p

[macdonst]

@dompuiu I'm going to mark this as a duplicate of #9 even though your's was the earlier issue. Should have a fix this week.