Causal error messages are not great!

brian@DESKTOP-CODN6ST MINGW64 /c/Users/brian/Documents/dev/taskcluster-crater/rs
$ rustup update stable
info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
info: downloading component 'rustc'
info: downloading component 'rust-std'
info: downloading component 'cargo'
info: downloading component 'rust-std' for 'armv7-apple-ios'
info: downloading component 'rust-std' for 'i686-apple-darwin'
error: component download failed for rust-std-i686-apple-darwin
info: caused by: could not download file from 'https://static.rust-lang.org/dist/2017-03-11/rust-std-1.16.0-i686-apple-darwin.tar.gz' to 'C:\Users\brian\.rustup\tmp\4y0bss25meytiiyq_file
info: caused by: error during download
info: caused by: [18] Transferred a partial file (transfer closed with 3544136 bytes remaining to read)

Here it's on the 4th line back that we're told there's an error. All subsequent lines just claim to be 'info', and the info lines contain double-colons!

This needs some custom formatting that clearly indicates all this is error stuff.

Start by reproducing the shown rustup update stable output and inspect the rustup error-reporting path that formats the causal chain. Done means the initial failure and subsequent caused-by lines are clearly presented as error output rather than misleading info lines.