Bad error when downloading bogus versions

[brson]

1 brian@ip-10-235-20-183:/mnt/dev/hello⟫ rustup update 1.0.0-alpha
info: syncing channel updates for '1.0.0-alpha'
info: downloading component 'rust'
error: could not download file from 'https://static.rust-lang.org/dist/rust-1.0.0-alpha.tar.gz.sha256' to '/home/brian/dev/.multirust/tmp/3ujwchag_1tpheuu_file
info: caused by: Status: 404 Not Found

This should say something more like "no release found for X".

[brson]

This should be easy. Just detect the Download404 error on this code path, then call chain_err on it to give it a more useful message.

[cramertj]

I missed this at the time but saw it pop up in "This Week in Rust." I'd like to give it a shot, if possible.