core: emit auto-auth errors

[stephenplusplus]

⚠️ Opinions needed on this one.

We have been ignoring the error that google-auth-library returns when it can't find the environment's default credentials.

As explained in the comment from the code:

google-auth-library returns a "Could not load..." error if it can't get an access token. However, it's possible an API request doesn't need to be authenticated, e.g. when downloading a file from a public bucket. We consider this error a warning, and allow the request to go through without authentication, relying on the upstream API to return an error the user would find more helpful, should one occur.

We have a bug currently because of this: GoogleCloudPlatform/nodejs-getting-started#25 - I'll explain this in a comment on the offending line of code.

Considering this instantiation:

var gcloud = require('gcloud');

// No credentials provided:
var dataset = gcloud.datastore().dataset({
  projectId: 'grape-spaceship-123'
});

Here is what will happen if we merge this PR:

dataset.runQuery(/*...*/, function(err) {
  // The error comes from google-auth-library:
  // err.message = 'Could not load the default credentials. Browse to https://developers.google.com/accounts/docs/application-default-credentials for more information.'
});

Pretending we didn't merge this PR, and just fixed the bug mentioned earlier, here's what would happen:

dataset.runQuery(/*...*/, function(err) {
  // The error comes from the API:
  // err.message = 'Unauthorized'
});

To get the better error (the first example), we have to not allow unauthenticated requests. I'm not sure if/what/how much of our API allows unauthenticated requests, or if we even want to support that in the first place?

[stephenplusplus]

Let's just go with it.