Improve error for nightly-only '--message-format' args

[matthewhughes934]

Previously on a stable build (json is only supported in nightly builds):

$ cargo fmt --message-format json
Invalid value for `--emit` - using an unstable value without `--unstable-features`

With this change:

$ CFG_RELEASE_CHANNEL=stable cargo make bin-build
$ PATH="./target/debug:$PATH" cargo fmt --message-format json
--message-format json is only supported in nightly builds
This utility formats all bin and lib files of the current crate using rustfmt.
<-- SNIP: the rest of the usage follows -->

This behaviour is similar to how args to --emit are handled in rustfmt.

This change also drops displaying json as an option to --message-format on stable builds:

$ PATH="./target/debug:$PATH" cargo fmt --help | grep --max-count 1 --after-context 1 message-format
    --message-format <message-format>
        Specify message-format: short|human

This required rewriting the check for nightly builds (copying from the one in src/release_channel.rs) to be const so I could rewrite the --message-format flag's help string.

Issue: #6752