Error handling with node-fetch

Because window.fetch isn't designed to transparent about the cause of request errors, we have to come up with our own solutions.

The basics:

All operational errors are rejected as FetchError, you can handle them all through promise catch clause.
All errors comes with err.message detailing the cause of errors.
All errors originated from node-fetch are marked with custom err.type.
All errors originated from Node.js core are marked with err.type = system, and contains addition err.code and err.errno for error handling, they are alias to error codes thrown by Node.js core.
Programmer errors are either thrown as soon as possible, or rejected with default Error with err.message for ease of troubleshooting.

List of error types:

Because we maintain 100% coverage, see test.js for a full list of custom FetchError types, as well as some of the common errors from Node.js