Add an easy way to check the error message

[madsmtm]

In rust-lang/rust#147967, I and several others were confused by a bisection because the compiler changed in other ways in the regression range that also broke building the repository.

I know that we could have avoided that with something like:

#!/bin/sh
cargo check 2>&1 | grep "message"

$ cargo bisect-rustc --script=./test.sh

But that's a bit of a mouthful. Do you think it would be worthwhile to add a --message parameter to make this easier?

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Alternatively, or maybe additionally, cargo-bisect-rustc could by default parse the error message(s) from the end bisection, and ensure that it stays the same across all the "inner" invocations. Or at least that it warns if they changed.