Be more specific if target was not found or not executable

[technicalpickles]

❯ scope doctor run
10:38:30  INFO More detailed logs at /tmp/scope/scope-root-20240123-vK6v.log
10:38:30 ERROR Critical Error. File /Users/josh.nichols/workspace/myapp/.scope/bin/rbenv-configured.sh was not executable or it did not exist.

The file does exist, but it's not executable. It should be possible to check if it exists, then if it's executable to give a better message for people working on these.

[rubberduck203]

Confirmed this still reproduces against current `main` (2026-07-28).

`check_pre_exec` in `src/shared/capture.rs` (lines 289-318) still collapses three distinct failure modes into the same generic `CaptureError::MissingShExec` message ("File {name} was not executable or it did not exist."):

1. the binary can't be resolved on `PATH` (`which_in` fails)
2. the resolved path doesn't exist on disk
3. the file exists but lacks the executable permission bit (checked around line 310-314)

The code is essentially unchanged since this was filed, so distinguishing these three cases in the error message (or splitting into distinct error variants) would resolve this directly.