Error handling with node-fetch

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

The basics:

A cancelled request is rejected with an AbortError. You can check if the reason for rejection was that the request was aborted by checking the Error's name is AbortError.

fetch(url, { signal }).catch(err => {
  if (err.name === 'AbortError') {
    // request was aborted
  }
})

All operational errors other than aborted requests are rejected with a FetchError. You can handle them all through the promise catch clause.
All errors come with an err.message detailing the cause of errors.
All errors originating from node-fetch are marked with a custom err.type.
All errors originating from Node.js core are marked with err.type = 'system', and in addition contain an err.code and an err.errno for error handling. These are aliases for 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

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