Implement fallout strategy failed individual message

Develop a fallout strategy for individual datastream messages with
datastream interface mapping. Retention types include 'volatile,' stored
in cache memory, and 'stored,' in the local database. The default
retention value is 'discarded,' and the 'discard' message retention
type does not save.

Signed-off-by: Osman Hadzic <osman.hadzic@secomind.com>

AstarteDeviceSDKCSharp/Transport/MQTT/AstarteMqttV1Transport.cs

@@ -46,7 +46,7 @@ public AstarteMqttV1Transport(MutualSSLAuthenticationMqttConnectionInfo connecti
         public override async Task SendIndividualValue(AstarteInterface astarteInterface,
         string path, object? value, DateTime? timestamp)
         {
-            AstarteInterfaceDatastreamMapping mapping;
+            AstarteInterfaceDatastreamMapping mapping = new();
             int qos = 2;
 
             if (astarteInterface.GetType() == (typeof(AstarteDeviceDatastreamInterface)))
@@ -67,7 +67,15 @@ public override async Task SendIndividualValue(AstarteInterface astarteInterface
             string topic = _baseTopic + "/" + astarteInterface.InterfaceName + path;
             byte[] payload = AstartePayload.Serialize(value, timestamp);
 
-            await DoSendMqttMessage(topic, payload, qos);
+            try
+            {
+                await DoSendMqttMessage(topic, payload, qos);
+            }
+            catch (MqttCommunicationException ex)
+            {
+                HandleDatastreamFailedPublish(ex, mapping, topic, payload, qos);
+            }
+
         }
 
         private async Task DoSendMqttMessage(string topic, byte[] payload, int qos)
@@ -94,6 +102,7 @@ private async Task DoSendMqttMessage(string topic, byte[] payload, int qos)
             catch (Exception)
             {
                 _astarteTransportEventListener?.OnTransportDisconnected();
+                throw new MqttCommunicationException(topic);
             }
 
         }
@@ -156,7 +165,6 @@ public override async Task SendAggregate(AstarteAggregateDatastreamInterface ast
             }
             catch (MqttCommunicationException e)
             {
-
                 HandleDatastreamFailedPublish(e, mapping, topic, payload, qos);
             }
         }