Two lines of 'error:' when a rust-toolchain file is invalid

[jyn514]

This is not the same as #2541 - this is two lines, not error: twice in one line.

Problem

$ cargo check
error: invalid channel name 'use x.py instead: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html' in '/home/joshua/rustc/rust-toolchain'
error: caused by: invalid toolchain name: 'use x.py instead: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html'

This is the same error twice, there's no need for both.

Steps

1. echo invalid-toolchain > rust-toolchain
2. cargo check

Possible Solution(s)

Notes

Output of rustup --version: rustup 1.22.1 (b01adbbc3 2020-07-08)
Output of rustup show:

Default host: x86_64-unknown-linux-gnu
rustup home:  /home/joshua/.local/lib/rustup

... about 20 toolchains ...

active toolchain
----------------

(error: invalid channel name 'use x.py instead: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html' in '/home/joshua/rustc/rust-toolchain', invalid toolchain name: 'use x.py instead: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html')

[jyn514]

See also rust-lang/rust#79021, which means people will see this error a lot. That PR was closed.