Bluetooth: Mesh: fix FW Update procedure recovery on on DFU Server side

If device rebooted while in `BT_MESH_DFU_PHASE_TRANSFER_ACTIVE` phase
we should not recover into it, but set `BT_MESH_DFU_PHASE_TRANSFER_ERR`
instead. It's because we no longer remember which block is currently
transfered so new FW Update Start must be received and transfer
restarted.

If device rebooted while in `BT_MESH_DFU_PHASE_VERIFY_OK` it means
we didn't manage to apply image before reboot. We should enter
`BT_MESH_DFU_PHASE_VERIFY_FAIL` phase, which will allow to verify it
again and possibly apply. This is with agreement to specification
regarding receiving FW Update Start when FW is already received.

This commit also alligns handle_start with specification: when
FW is already received, we should set phase to
`BT_MESH_DFU_PHASE_VERIFY`, send Status message, then proceed to Verify.
Previously, `verify` was called before sending status, so status
contained phase set to `BT_MESH_DFU_PHASE_VERIFY_OK` or
`BT_MESH_DFU_PHASE_VERIFY_FAIL`. This would prevent DFU Client
enter refresh procedure.

`blob_recover` also expects BT_MESH_DFU_PHASE_TRANSFER_ERR now.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>