[pwrmgr] Error FSM state is not observable in pwrmgr_fsm

### Description

## Description

The pwrmgr_fsm implements a sparsely encoded state machine. All deviations from the valid state, e.g., due to a fault on the state register, lead the FSM to transfer to the `default` state.

However, compared to other FSMs, the particular FSM does not implement a dedicated error signal for this FSM. Consequently, it is impossible to observe if the error state is entered.

## Potential Solution

Implement a dedicated error signal for the FSM that asserts if the FSM reaches the default state.

Start by locating the pwrmgr_fsm implementation and compare it with other FSMs that expose a dedicated error signal. Add an observable signal that asserts when the FSM enters its default state, then verify that invalid-state behavior and the signal are covered by the existing pwrmgr tests.

Written by the indexing model from the issue text.