Improve network error handling during SDK setup #137

The "installing toolchain" step occurs after we unxip. If toolchain download fails, the user may be left with a cryptic error message. There are a few issues here:

1. Users might assume this is due to their Xcode.xip being broken, so they end up re-downloading the multi-gigabyte xip to no avail. We should clearly tell the user this error actually occurred when downloading the Darwin toolchain from GitHub.
2. We waste time unxipping before we initiate the connection to download the Darwin toolchain. If we flipped the steps around, it would tighten the iteration loop.