enhance(build/spas): allow yarn dev without internet if DEV_MODE is enabled

[caugner]

Summary

(MP-899)

Problem

When working on a train or a plane without internet, you cannot run yarn dev, because it fails when building the SPA.

Solution

Convert the errors to warnings if DEV_MODE is enabled (via setting REACT_APP_DEV_MODE=true in the .env).

Note: Technically, yarn tool popularities is also affected, but the command defaults to not downloading the popularities again if they already exist (unless --refresh is provided), so this PR doesn't cover that case (just don't delete your popularities.json when you're on a plane 🙂).

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Screenshots

Before

% yarn tool spas
yarn run v1.22.21
$ NODE_OPTIONS='--no-warnings=ExperimentalWarning --loader ts-node/esm' node ./tool/cli.ts spas

error: getaddrinfo ENOTFOUND api.github.com

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

After

% yarn tool spas
yarn run v1.22.21
$ NODE_OPTIONS='--no-warnings=ExperimentalWarning --loader ts-node/esm' node ./tool/cli.ts spas
Warning: Couldn't fetch recent GitHub contributions for repo mdn/translated-content!
Warning: Couldn't fetch recent GitHub contributions for repo mdn/content!
Warning: Couldn't fetch hacks.mozilla.org feed!
Built 146 SPA related files
✨  Done in 17.81s.

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How did you test this change?

Ran yarn tool spas with wifi disabled (see above).

[argl]

Looks good to me