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