Confusing error message when using `--keep-stage-std` after cleaning

[Noratrieb]

./x.py clean
./x.py build --keep-stage-std 0

This of course doesn't work, as there is no stage to keep.

but the error message is less than ideal

arning: Using a potentially old libstd. This may not behave well.
Copying stage0 library from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
thread 'main' panicked at 'fs::read(stamp) failed with No such file or directory (os error 2) ("/home/nilsh/projects/rust/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/.libstd.stamp")', lib.rs:1435:24
note: run with `RUST_BACKTRACE=1 environment variable to display a backtrace
Build completed unsuccessfully in 0:00:00

Since the keep-stage was behind an alias that I forgot I used I was quite confused with this, since this seemed like an internal bootstrap error and not a user error on my side.

[Teapot4195]

@rustbot claim

[jyn514]

cc #107019