Handle Github rate-limiting response in download-experimental-build.js

[blakef]

Summary

When running the download-experimental-build.js, fetching the build-id for the commit uses a Github API. The code currently assumes that failure is due to the commit not existing. I hit rate limiting (working over a VPN), but the error reported the commit could not be found for main.

This change surfaces the API error message should it not be a 404, in which case the previous error message worked well:

How did you test this change?

Ran the change while I was being rate limited:

❯ ./download-experimental-build.js --commit=main
Error: API rate limit exceeded for XXX.XXX.XXX.XXX. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)
        https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting

Confirmed that I was actually being rate limited:

❯ curl -I https://github.com/api/users/octocat 2>&1 | grep remaining
x-ratelimit-remaining: 0

Ran the change when I wasn't being rate limited:

❯ ./download-experimental-build.js --commit=foobar
Error: Could not find commit for: foobar

Ran the change on my local machine while I was being rate limited. Waited for the

[sizebot]

Comparing: 357a613...f5611e9

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	131.49 kB	131.49 kB	=	42.15 kB	42.15 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	136.73 kB	136.73 kB	=	43.69 kB	43.69 kB
facebook-www/ReactDOM-prod.classic.js	=	441.13 kB	441.13 kB	=	80.41 kB	80.41 kB
facebook-www/ReactDOM-prod.modern.js	=	426.34 kB	426.34 kB	=	78.23 kB	78.23 kB
facebook-www/ReactDOMForked-prod.classic.js	=	441.13 kB	441.13 kB	=	80.42 kB	80.42 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against f5611e9

[acdlite]

Seems fine, thanks!