don't throw alerts in the middle of someone's presentation

www/slide/main.js

@@ -635,8 +635,10 @@ define([
                     toolbar.reconnecting(info.myId);
                     Cryptpad.findOKButton().click();
                 } else {
+                    if (!Slide.isPresentURL()) {
                         Cryptpad.alert(Messages.common_connectionLost, undefined, true);
                     }
+                }
             };
 
             config.onError = onConnectError;