Provide better error messages for common auth errors

[silvolu]

@ryanseys suggested to add more sanity checking in #261 (comment), but I'm not sure we can do more than checking that the .json file or the credentials object contains the expected fields. #261 (comment) shows that it doesn't solve the problem of a developer using a non-key value for private_key when passing in a credentials object.
I think we could at least intercept the error and give back a more meaningful explanation, something like "There is something wrong with your key. If you are using a .json file, make sure that on the Google Developer Console you've generated the file for a ClientId of Service Account type....."

[stephenplusplus]

I think we need to solve this problem on the docs level. (Yeah, I know we're trying pretty hard!) The problem here is, if I pass a faulty private_key, it is going to attempt to be signed via jws, where an error is thrown.

We would have to try/catch authorizing API requests. Should we open an issue on node-gapitoken to use a try catch around the sign, or let it go?

[silvolu]

Let's wait for @ryanseys new thing.

[stephenplusplus]

@ryanseys how does your lib handle this situation? Are we resolved?

[ryanseys]

gtoken surrounds the signing of the JWT with a try/catch and it will return the error thrown in the callback if the signing fails. gtoken is designed to never throw. If it does, that's a bug and I should be punished accordingly (and you should open an issue). You can see that specific case here.

[stephenplusplus]

And this is where I return that error, if there was one? https://github.com/stephenplusplus/google-service-account/blob/master/connection.js#L72

[ryanseys]

Yes sir.