Include better context for "already exists" error in compiletest
I encountered the following error from `x.py test tests/ui` today.
```console
---- [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:
```console
---- [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*.
cdfaf69498