fix: 🐛 avoid "A server error occurred" msgs related to polling

Polling request might time out if app goes into background. New polling
request will be started when app comes back into foreground. There is no
reason to bother the enduser with messages related to polling. Hence, in
this commit `PollMessageDelivery` will no longer emit error messages
related to polling.

Sources/SAPCAI/Foundation/Networking/PollMessageDelivery.swift

@@ -24,7 +24,11 @@ public class PollMessageDelivery: MessageDelivering {
 
     private var conversationId: String?
 
-    private var state: State = .stopped
+    private var state: State = .stopped {
+        didSet {
+            self.logger.debug("Polling \(state)")
+        }
+    }
 
     private var logger = Logger.shared(named: "PollMessageDelivery")
 
@@ -100,16 +104,13 @@ public class PollMessageDelivery: MessageDelivering {
                 }
 
             case .failure(let error):
+                self.logger.error(error.debugDescription, error: error)
                 switch error.type {
                 case .server:
-                    self.logger.error(error.debugDescription, error: error)
-                    self.onMessages?(.failure(error))
                     self.startPolling()
                 case .cancelled:
-                    () // this naturally occurs when bot responses early?
+                    ()
                 case .dataDecoding, .conversationNotFound:
-                    self.logger.error(error.debugDescription, error: error)
-                    self.onMessages?(.failure(error))
                     self._stop()
                 }
             }