Work around devices without end model.

Some devices like SMA STP10.0-3SE-40 do not have an end model and discovery fails. Work around that by pretending an end model was found when an IllegalDataAddress error occurs and at least one valid model has been found before.

src/client/async.rs

@@ -150,16 +150,28 @@ async fn discover_models(
 
     let mut models = Models::default();
     let mut unknown_models: Vec<UnknownModel> = vec![];
+    let mut model_count = 0;
 
     loop {
-        let [model_id, len] = apply_timeout(
-            read_holding_registers_array::<2>(client, addr),
-            read_timeout,
-        )
-        .await?;
+        let res = apply_timeout(read_holding_registers_array::<2>(client, addr), read_timeout).await;
+        let [model_id, len] = match res {
+            Ok(d) => d,
+            Err(ModbusError::IllegalDataAddress) => {
+                if model_count > 0 {
+                    [0xFFFF, 0] //simulate end for devices without end model
+                } else {
+                    return Err(DiscoveryError::ModbusError(ModbusError::IllegalDataAddress));
+                }
+            }
+            Err(e) => return Err(DiscoveryError::ModbusError(e)),
+        };
+
         if model_id == 0xFFFF {
             break;
         }
+
+        model_count += 1;
+
         addr = addr.checked_add(2).ok_or(DiscoveryError::AddressOverflow)?;
         if !models.set_addr(model_id, addr, len) {
             unknown_models.push(UnknownModel {