Improve the Error handling sample and add more documentation

[trollkotze]

https://github.com/googleapis/nodejs-datastore/blob/master/samples/error.js

It would be good to have actual example of error handling, and/or complete documentation about what errors can occur and what they mean.

[stephenplusplus]

I believe the upstream docs site should cover general error handling advice. Is this the type of information you're looking for: https://cloud.google.com/datastore/docs/concepts/errors? If so, we can link out to that page somewhere within the repo.

@alexander-fenster do you know where this sample actually shows up on the docs site?

[trollkotze]

Thank you! That is useful. On my first encounter with that page I had not looked deep enough and thought these were only HTTP responses for the REST-API, without one-to-one correspondence to errors inside the client libs. Mea culpa!
Anyway, would be clearer if the client library's documentation and error handling sample would include these as well.