Does not return a non-zero exit status when update fails

If an update fails, rustup update still returns a zero exit status. I think it should return a non-zero exit status so that if it's called from another program or script, that program or script can easily determine that it failed without having to scrape its output.

Here's an example of the messages that are shown when the update fails:

info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
info: latest update on 2017-07-20, rust version 1.19.0 (0ade33941 2017-07-17)
info: downloading component 'rustc'
info: downloading component 'rust-std'
info: downloading component 'cargo'
info: downloading component 'rust-docs'
info: removing component 'rustc'
info: removing component 'rust-std'
info: rolling back changes
error: could not rename component file from 'C:\Users\kraaimx\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/x86_64-pc-windows-msvc/lib/arena-12c6b2edb037e265.dll.lib' to 'C:\Users\kraaimx\.rustup\tmp\9lq7jdzdcw367gzh_file'
info: syncing channel updates for 'nightly-x86_64-pc-windows-msvc'
info: latest update on 2017-07-20, rust version 1.20.0-nightly (582af6e1a 2017-07-19)
info: downloading component 'rustc'
info: downloading component 'rust-std'
info: downloading component 'cargo'
info: downloading component 'rust-docs'
info: removing component 'rustc'
info: removing component 'rust-std'
info: removing component 'cargo'
info: removing component 'rust-docs'
info: installing component 'rustc'
info: installing component 'rust-std'
info: installing component 'cargo'
info: installing component 'rust-docs'
info: checking for self-updates

  stable-x86_64-pc-windows-msvc update failed - rustc 1.18.0 (03fc9d622 2017-06-06)
       nightly-x86_64-pc-windows-msvc updated - rustc 1.20.0-nightly (582af6e1a 2017-07-19)

Start by tracing the rustup update command's handling of a failed channel update and how it selects the final process status. Reproduce a mixed result like the report, where one toolchain fails and another succeeds, then verify that the command returns non-zero while still reporting each update result.