Bluetooth: Mesh: FU Server should not update internal state on error

According to conditions in the DFU spec section 6.1.3.7 (Sending a
Firmware Update Status message) and in 'Idempotency' part of section
6.1.3.4 (Receiving a Firmware Update Start message), the FU Server
should not update internal state when 'Wrong Phase' error condition
is triggered.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>