Fix creation of invalid librusty_v8.a

[wg]

Building rusty_v8 for targets lacking a binary librusty_v8.a, for example aarch64-unknown-linux-musl, currently fails with a confusing error message:

error: failed to add native library $PWD/target/aarch64-unknown-linux-musl/debug/gn_out/obj/librusty_v8.a: file too small to be an archive

This is because build.rs falls back to use of curl after download_file.py fails. The curl command is currently ignoring HTTP error statuses and writes the HTTP error message to librusty_v8.a:

% cat $PWD/target/aarch64-unknown-linux-musl/debug/gn_out/obj/librusty_v8.a
Not Found

This PR adds curl's -f flag to the command arguments which results in a more useful error message, as well as not writing an invalid librusty_v8.a.

[ry]

Thanks!