is_available() only caught FileNotFoundError and TimeoutExpired. On WSL,
a /mnt/c/.../WindowsApps entry on $PATH returns EACCES during exec, and
Python raises PermissionError. That escaped is_available() and crashed
pipeline.diagnose() before any source ran.
Catch OSError instead. It covers FileNotFoundError, PermissionError, and
any other spawn-time OS error, so a non-executable xurl on PATH falls
through to the next backend instead of aborting the run.
Ilia Alshanetsky