use proper error handling

rust/src/tank.rs

@@ -123,6 +123,12 @@ impl TankState {
         }
     }
 
+    pub fn got_error(&self, config: &TankConfig) -> Option<TankError> {
+        match self {
+            TankState::TankSensorPresent(raw_value_mv) => raw_voltage_to_tank_fill_percent(*raw_value_mv, config).err(),
+            TankState::TankSensorDisabled => Some(TankError::SensorDisabled),
+        }
+    }
 
 }