errors: Reorder component-unavailable message parts

[kinnison]

The ordering of some of the parts of the component-unavailable
error message was unfortunate, resulting in confusion. This
reorders those so that errors are slightly clearer.

Fixes: #1768

As an example, the old message form would be:

error: some components unavailable for download: 'rls', 'clippy'
if you require these components, please install and use the latest successful build version, which you can find at https://rust-lang-nursery.github.io/rust-toolstate, for example.
rustup install nightly-2018-12-27 for channel 'nightly'

The new form is:

error: some components unavailable for download: 'rls', 'clippy' for channel 'nightly'
if you require these components, please install and use the latest successful build version, which you can find at https://rust-lang-nursery.github.io/rust-toolstate, for example.
rustup install nightly-2018-12-27

/cc @vi

[vi]

if you require

"If you require" (capitalized)?

for example.

"for example:" (colon)? Or is it not about the command line, but about URL to look for build status? Then there should be some other introductory sentence before the command line sample, such as after determining the required date, use command line like this:.

[kinnison]

Fair, I'll get that message updated later. Is this the only problem you have with the result?

[vi]

Probably yes, although I'd also specified explicitly that it would not be the nightly toolchain.

After determining the date, install it with a command like this:

    rustup install nightly-2018-12-27

Then use it as a toolchain name:

    cargo +nightly-2018-12-27 build

The system (tracking nightly which points to some date, plus explicit dated nightlies) is in general a bit confusing, so longer help messages won't hurt.

[kinnison]

Explanatory message updated to:

If you require these components, please install and use the latest successful build version,
which you can find at https://rust-lang-nursery.github.io/rust-toolstate.

After determining the correct date, install it with a command such as:

    rustup toolchain install nightly-2018-12-27

Then you can use the toolchain with commands such as:

    cargo +nightly-2018-12-27 build

[vi]

Looks OK.

If #1628 is implemented some way (e.g. --override-nightly-date option for rustup update nightly), then this message should be updated.

[kinnison]

Thanks @vi