OAuth: Handle errors in redirect when passed by GitHub

[petr-muller]

Make Prow properly detect and log possible error information when passed
by GitHub and differentiate the messaging from the case where token
exchange fails for other reasons.

Certain error conditions [1] cause GitHub to call back the OAuth
redirect with error information present in parameters. Previously,
Prow's handler assumed the token to be always present, failing with a
generic error message when it was not.

[1] https://developer.github.com/apps/managing-oauth-apps/troubleshooting-authorization-request-errors/

Fixes: #10048

[k8s-ci-robot]

Hi @petr-muller. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[fejta]

/ok-to-test
/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 45f94d16e72bae4889d0908ce2973c5d3db53d78

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fejta, petr-muller

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