Plugin VM installer: Show network error if download size is smaller than expected

Normally a failed download should result in DOWNLOAD_FAILED_NETWORK or
a similar error, but in some cases we only detect this when hashing the
download. If the hash doesn't match but the downloaded image was smaller
than expected, show a network error instead of a hash error.

Bug: b/197278759
Change-Id: I364b5b5848b8ef162e6e4a0a05885d21e4d41dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3139735


Commit-Queue: Timothy Loh <timloh@chromium.org>
Reviewed-by: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/main@{#918703}