`codegen-backends = ["rustc_codegen_cranelift"]` should give a better error message

[jyn514]

I tried this code:

echo 'codegen-backends = ["rustc_codegen_cranelift"]` >> config.toml
x test ui

I expected to see this happen: An error saying that rustc_codegen_cranelift is invalid and should be cranelift instead.

Instead, this happened:

Building stage0 codegen backend rustc_codegen_cranelift
error: manifest path `/home/jyn/src/rust2/compiler/rustc_codegen_rustc_codegen_cranelift/Cargo.toml` does not exist

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HEAD is branched from 13afbda

[onur-ozkan]

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