Doc/powermeter error handling

doc/ocmf/powermeter_start_transaction.md

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+```mermaid
+sequenceDiagram
+autonumber
+participant Powermeter
+participant EvseManager
+participant OCPP
+participant CSMS
+
+title Start of a Transaction
+
+Note over EvseManager: User plugs in EV and authorizes
+
+EvseManager->>OCPP: Event(SessionStarted)
+
+OCPP->>CSMS: StatusNotification.req(Preparing)
+CSMS-->>OCPP: StatusNotification.conf
+
+alt successful case
+    EvseManager->>Powermeter: startTransaction
+    Powermeter-->>EvseManager: startTransaction Response (OK/ID)
+
+    EvseManager->>OCPP: Event(TransactionStarted)
+    OCPP->>CSMS: StartTransaction.req
+    CSMS-->>OCPP: StartTransaction.conf
+
+    Note over EvseManager: Transaction started successfully
+
+else startTransaction failing due to power loss
+    EvseManager->>Powermeter: startTransaction
+    Powermeter-->>EvseManager: startTransaction Response (FAIL)
+
+    EvseManager->>OCPP: Event(Deauthorized)
+
+    OCPP->>CSMS: StatusNotification.req(Finishing)
+    CSMS-->>OCPP: StatusNotification.conf
+
+    EvseManager->>OCPP: raiseError (PowermeterTransactionStartFailed)
+    OCPP->>CSMS: StatusNotification.req(Finishing, PowermeterTransactionStartFailed)
+    CSMS-->>OCPP: StatusNotification.conf
+
+    Note over EvseManager: Transaction did not start
+end
+
+alt EvseManager configured to become inoperative in case of Powermeter CommunicationError
+    Powermeter->>EvseManager: raise_error(CommunicationError)
+    Note over Powermeter,EvseManager: Powermeter raises a CommunicationError <br/>and EvseManager is registered for notification
+    EvseManager->>OCPP: raise_error (Inoperative)
+    OCPP->>CSMS: StatusNotification.req(Faulted)
+    CSMS-->>OCPP: StatusNotification.conf
+end
+
+```

doc/ocmf/powermeter_stop_transaction.md

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+```mermaid
+sequenceDiagram
+autonumber
+participant Powermeter
+participant EvseManager
+participant OCPP
+participant CSMS
+
+title Stopping Transaction in Error
+
+Note over Powermeter, CSMS: Transaction is running
+
+Powermeter->>Powermeter: detects a <br/> CommunicationError
+Note over Powermeter,EvseManager: Powermeter raises a CommunicationError <br/>and EvseManager is registered for notification
+Powermeter->>EvseManager: raise_error (CommunicationFault)
+Powermeter->>OCPP: raise_error (CommunicationFault)
+
+OCPP->>CSMS: StatusNotification.req(Charging, CommunicationFault)
+CSMS-->>OCPP: StatusNotification.conf
+
+alt EvseManager configured to become inoperative in case of PowermeterCommError
+    EvseManager->>EvseManager: Pause charging
+    EvseManager->>OCPP: raiseError (Inoperative)
+    OCPP->>CSMS: StatusNotification.req(Faulted)
+    Note over EvseManager: Note that we just continue charging otherwise
+end
+
+Note over Powermeter, CSMS: User stops the transaction
+
+alt successful case (Powermeter has no CommunicationError)
+    EvseManager->>Powermeter: stopTransaction (ID)
+    Powermeter-->>EvseManager: stopTransaction Response (OK/OCMF)
+    EvseManager->>OCPP: Event(TransactionFinished(OCMF))
+
+    OCPP->>CSMS: StopTransaction.req(OCMF)
+    CSMS-->>OCPP: StopTransaction.conf
+else stopTransaction failing due to subsequent power loss (this applies as well when Powermeter still in CommunicationError)
+    EvseManager->>Powermeter: stopTransaction (ID)
+    Powermeter->>EvseManager: stopTransaction Response (FAIL)
+    EvseManager->>OCPP: Event(TransactionFinished)
+
+    Note right of OCPP: In this case we can't stop the transaction including the OCMF
+    OCPP->>CSMS: StopTransaction.req()
+    CSMS-->>OCPP: StopTransaction.conf
+end
+```