"incus storage volume import": name clash gives an unfriendly SQL error

[candlerb]

Required information

Incus 0.6, Ubuntu 22.04

Issue description

Importing a storage volume where the name already exists gives a SQL uniqueness constraint error.

It would be better if it said "a volume with this name already exists", or similar.

Steps to reproduce

1. Export a storage volume
2. Immediately re-import it without giving it a different name.

$ incus storage volume import default default_testzfs.tgz
Error: Create custom volume from backup: Error inserting volume "testzfs" for project "default" in pool "default" of type "custom" into database "UNIQUE constraint failed: index 'storage_volumes_unique_storage_pool_id_node_id_project_id_name_type'"

Of course, it's fine if you choose a different name:

$ incus storage volume import default default_testzfs.tgz test1
$