tool/gocross: fail if the toolchain revision isn't findable

This used to make sense, but after a refactor somewhere along the line
this results in trying to download from a malformed URL and generally
confusing failures.

Signed-off-by: David Anderson <danderson@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f