spurious error when selecting default toolchain

[Lilith River]

If a toolchain is already the default, selecting it in Tokomak causes the following error:

info: syncing channel updates for 'nightly-2017-03-01-x86_64-unknown-linux-gnu '
error: target not found: 'x86_64-unknown-linux-gnu '

The work-around is to pick a different toolchain, then go back to the one you want.

This is only necessary during initial install.