notifications: Add RetryingDownload notification
notifications: Add RetryingDownload notification

This is to be used when the downloader decides it has to retry
a download (for example, if it timed out, the socket died, etc)

Signed-off-by: Daniel Silverstone <3cc17a76d780e0cd3d87bad586a1c53967be5625@digital-scurf.org>

nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs