Check if a nightly build for the given date actually exists before attempting to install

$ rustup --version
rustup 0.6.3 (a0e2132 2016-08-28)

$ rustup toolchain install nightly-2016-10-13
info: syncing channel updates for 'nightly-2016-10-13-x86_64-unknown-linux-gnu'
error: no release found for 'nightly-2016-10-13'

$ rustup toolchain install nightly
info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
info: downloading component 'rustc'
 37.2 MiB /  37.2 MiB (100 %)   1.4 MiB/s ETA:   0 s
info: downloading component 'rust-std'
 44.5 MiB /  44.5 MiB (100 %)   1.6 MiB/s ETA:   0 s
info: downloading component 'rust-docs'
  9.2 MiB /   9.2 MiB (100 %)   1.4 MiB/s ETA:   0 s
info: downloading component 'cargo'
  3.9 MiB /   3.9 MiB (100 %)   1.1 MiB/s ETA:   0 s
info: installing component 'rustc'
info: installing component 'rust-std'
info: installing component 'rust-docs'
info: installing component 'cargo'

  nightly-x86_64-unknown-linux-gnu installed - rustc 1.14.0-nightly (098d22845 2016-10-13)

The most recent nightly I'm able to install by date is 2016-10-11.

Start by reproducing rustup toolchain install nightly-2016-10-13 and trace the toolchain installation path that handles dated channels. Compare it with installation of nightly; done means a missing dated nightly is detected before component downloads and produces a clear error, while an existing nightly remains installable.