fs_mgr: trigger dm-verity error handling for invalid signatures

Currently, the device doesn't mount verified partitions if the
verity table signature is invalid, which usually means it fails to
boot. This changes instead set up dm-verity with an invalid root
hash and triggers device-specific error handling to recover from
the corruption.

Change-Id: I6d693306fa0e7459c5500b028e433df61ecea6fb