Improve offline install error handling. (#17153)

* Handle braindead firewalls that block HTTP HEAD requests.

Apparently there exist firewalls out there that allow HTTP GET requests
but block HTTP HEAD requests. This causes issues in our installer code
because we use HEAD requests to efficiently check if files exist on
remote servers.

In most cases, correct behavior here is to check if a known valid URL
fails our remote file check, and if it does short-circuit the check on
subsequent calls.

* Bail early if we fail to download anything for offline installs.

This ensures we give a proper error message.

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