Surface umount stderr in UnmountError; user-friendly error messages in backup/close

[Copilot]

umount failures previously surfaced as an unhandled UnmountError stack trace with no context (e.g. "target is busy" when a shell is open on the device). This change propagates the umount stderr to the user as a plain error message.

shell_interface: add stderr to ShellInterfaceError

ShellInterfaceError now carries the stderr bytes from the underlying CalledProcessError, defaulting to b"" when output wasn't captured:

class ShellInterfaceError(RuntimeError):
    def __init__(self, msg: str, stderr: bytes = b"") -> None:
        super().__init__(msg)
        self.stderr = stderr

Both run_cmd and pipe_pass_cmd_to_real_cmd populate it on failure.

storage_device_managers: propagate stderr into UnmountError

unmount_device now runs umount with capture_output=True and attaches the decoded stderr directly to the UnmountError message — no more isinstance/__cause__ chain:

except sh.ShellInterfaceError as e:
    raise UnmountError(e.stderr.decode(errors="replace").strip()) from e

butter_backup CLI: catch UnmountError in backup and close

Both commands now catch UnmountError and print a human-readable message to stderr instead of letting the exception propagate. backup continues processing remaining devices and exits with code 1 after the loop if any unmount failed.