show 'try again' button when remote server wasn't found in previously active muc

src/main/java/eu/siacs/conversations/entities/Conversation.java

@@ -1007,6 +1007,18 @@ public class Conversation extends AbstractEntity implements Blockable, Comparabl
 		}
 	}
 
+	public int receivedMessagesCount() {
+		int count = 0;
+		synchronized (this.messages) {
+			for(Message message : messages) {
+				if (message.getStatus() == Message.STATUS_RECEIVED) {
+					++count;
+				}
+			}
+		}
+		return count;
+	}
+
 	private int sentMessagesCount() {
 		int count = 0;
 		synchronized (this.messages) {

src/main/java/eu/siacs/conversations/services/XmppConnectionService.java

@@ -2267,6 +2267,7 @@ public class XmppConnectionService extends Service {
 				public void onFetchFailed(final Conversation conversation, Element error) {
 					if (error != null && "remote-server-not-found".equals(error.getName())) {
 						conversation.getMucOptions().setError(MucOptions.Error.SERVER_NOT_FOUND);
+						updateConversationUi();
 					} else {
 						join(conversation);
 						fetchConferenceConfiguration(conversation);

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -1038,7 +1038,11 @@ public class ConversationFragment extends Fragment implements EditMessage.Keyboa
 					showSnackbar(R.string.joining_conference, 0, null);
 					break;
 				case SERVER_NOT_FOUND:
+					if (conversation.receivedMessagesCount() > 0) {
+						showSnackbar(R.string.remote_server_not_found,R.string.try_again, joinMuc);
+					} else {
 						showSnackbar(R.string.remote_server_not_found, R.string.leave, leaveMuc);
+					}
 					break;
 				case PASSWORD_REQUIRED:
 					showSnackbar(R.string.conference_requires_password, R.string.enter_password, enterPassword);
@@ -1059,6 +1063,7 @@ public class ConversationFragment extends Fragment implements EditMessage.Keyboa
 					showSnackbar(R.string.conference_shutdown, R.string.join, joinMuc);
 					break;
 				default:
+					hideSnackbar();
 					break;
 			}
 		} else if (account.hasPendingPgpIntent(conversation)) {