Don't try to create a Persistent Storage on a device that already has 2 partitions

Creating a Persistent Storage on a USB stick that already has a second partition fails in various ways, such as:

Oct 05 12:03:26 amnesia tpsd[6667]: INFO:device.py:346: [1] Initializing LUKS header
Oct 05 12:03:26 amnesia tpsd[6667]: INFO:device.py:361: [1] Executing command cryptsetup luksFormat --batch-mode --key-file=- --type=luks2 --pbkdf=argon2id --pbkdf-memory=1048576 --pbkdf-force-iterations=4 /dev/sda3
Oct 05 12:03:57 amnesia tpsd[16658]: WARNING: keyslots area (1015808 bytes) is very small, available LUKS2 keyslot count is very limited.
Oct 05 12:03:57 amnesia tpsd[16658]: Device /dev/sda3 is too small for activation, there is no remaining space for data.