broker: make the writer-error status message self-explanatory

When the writer fails mid-run the status carried only the raw writer_err
string. Prefix it with "Writer error, written data may be incomplete: "
so the operator sees the dataset may be truncated, not just a bare error
string. Priority order is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

broker/JFJochStateMachine.cpp

@@ -459,7 +459,7 @@ void JFJochStateMachine::MeasurementThread() {
                          BrokerStatus::MessageSeverity::Error);
             else if (!tmp_output.receiver_output.writer_err.empty())
                 SetState(JFJochState::Idle,
-                         tmp_output.receiver_output.writer_err,
+                         "Writer error, written data may be incomplete: " + tmp_output.receiver_output.writer_err,
                          BrokerStatus::MessageSeverity::Error);
             else if (tmp_output.receiver_output.writer_queue_full_warning)
                 SetState(JFJochState::Idle,