Throw a helpful error when fetch is not set

[oscard0m]

• Update entry in octokit.js README.md Proxy Servers (Node.js only) #2499
• Add section Fetch missing in octokit.js README.md #2500
• Update error message in request.js request.js#616

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Maybe we could add a section to the README and link to it from the error, so we can provide a more thorough explanation and most importantly we can update whenever needed.

I'd say the availability of fetch is independent of the Node version. fetch is still experimental and it can be disabled with --no-experiemental-fetch. Also Node is not the only place where Octokit is run, I'm not sure if e.g. Electron or React Native have a global fetch method.

I'd do something like this:

fetch is not set. Please pass a fetch implementation as new Octokit({ request: { fetch }}). Learn more at https://github.com/octokit/octokit.js/#fetch-missing

Originally posted by @gr2m in #2486 (comment)