Include better context for "already exists" error in compiletest

[dtolnay]

I encountered the following error from x.py test tests/ui today.

---- [ui] tests/ui/impl-trait/multiple-lifetimes/multiple-lifetimes.rs stdout ----
thread '[ui] tests/ui/impl-trait/multiple-lifetimes/multiple-lifetimes.rs' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 17, kind: AlreadyExists, message: "File exists" }', src/tools/compiletest/src/runtest.rs:134:43
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I found it impossible to unblock myself without knowing which directory it was stuck on.

Error message after this PR:

---- [ui] tests/ui/impl-trait/multiple-lifetimes/multiple-lifetimes.rs stdout ----
thread '[ui] tests/ui/impl-trait/multiple-lifetimes/multiple-lifetimes.rs' panicked at 'called `Result::unwrap()` on an `Err` value: failed to create output base directory /git/rust/build/x86_64-unknown-linux-gnu/test/ui/impl-trait/multiple-lifetimes/multiple-lifetimes

Caused by:
    File exists (os error 17)', src/tools/compiletest/src/runtest.rs:139:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Now I was able to run rm build/x86_64-unknown-linux-gnu/test/ui/impl-trait/multiple-lifetimes/multiple-lifetimes and unblock myself.

Seems to be related to #109509 moving tests/ui/impl-trait/multiple-lifetimes.rs to tests/ui/impl-trait/multiple-lifetimes/multiple-lifetimes.rs.

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 6b7b775 has been approved by Mark-Simulacrum

It is now in the queue for this repository.