Summary

Fix incus file mount and incus storage volume file mount in SSH SFTP listener mode to fail early when the entity (instance, storage pool, or storage volume) does not exist.
Previously, these commands could start the listener process even non-existent entities, resulting in confusing runtime behavior.

Resolves: #2944

Description

The sshSFTPServer doesn't require the entity directly as a parameter, they are used indirectly through the sftpConn lambda function.

These changes try to get the entity before calling sshSFTPServer and fail early if the entity does not exist.

Note: sftpConn is only called after an SSH connection, and sshSFTPServer doesn't return an error if sftpConn fails.

I also updated the help texts to clarify the listener behavior when the target path is omitted.

Behavior after changes

> incus file mount xxxxxx
Error: Failed to fetch instance "xxxxxx" in project "default": Instance not found
> incus storage volume file mount xxxxxx yyyyyy
Error: Storage pool not found
> incus storage volume file mount default yyyyyy
Error: Storage pool volume not found