Show specific error messages on errors creating the tunnel. Fixes #972

[metadaddy]

As described in #972, when creating a tunnel, cloudflared shows a somewhat cryptic error message on HTTP errors such as 429 (too many requests), since RunQuickTunnel() tries to parse the HTTP response body as JSON no matter the response status code.

This change detects non-200 status codes, shows a specific error for 429, and reports the status code for other HTTP errors.

Tests are included, and all existing and new tests pass.

[gitguardian]

️✅ There are no secrets present in this pull request anymore.

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[metadaddy]

I used old output from curl -X POST https://api.trycloudflare.com/tunnel as test data, which GitGuardian flagged, so I substituted clearly fake data, apart from the UUID. I'm not sure why GitGuardian is still complaining!

[metadaddy]

I removed the commit containing secret-looking test data and now GitGuardian is happy.

[metadaddy]

@DevinCarr Hi Devin - you seem to have been active in this part of cloudflared recently. Could I get a review on this PR? Thanks!

[GoncaloGarcia]

Thanks for doing this, I left a small comment, but looks good otherwise

[chungthuang]

Hi @metadaddy, thank you for making this contribution. We made a change to the quick tunnel file recently. Can you rebase?

[metadaddy]

@chungthuang Rebased and force-pushed.

@GoncaloGarcia Apologes - I missed your comment when you sent it. I added the HTTP response body to the error message as you requested.

[metadaddy]

@chungthuang / @GoncaloGarcia / @kornelski - I noticed that quick_tunnel.go had changed, so I rebased again.