Improve error format consistency

Currently errors come in either as:

• The error object in a callback, with a message attributes
• The error object in a callback, with a error-code and error-code-label attribute
• The response object in a callback, with a status attribute with a value of 0

This is causing a lot of headache when used (see nexmo/nexmo-cli#104).

I propose we instead return an error object and only an error object if an error occurred, and a response object and only a response object if no errors occurred. This would also bring the library more in line with how Promises work down the line if we want to go that way.

[pardel]

@cbetta better late than never 😉

[manchuck]

@cbetta If you update to V3, we are throwing errors as we have migrated to using promises. This should no longer be an issue for you.