Use the same disconnecter error translation key for all the apps

www/slide/main.js

@@ -782,7 +782,7 @@ define([
                 // inform of network disconnect
                 setEditable(false);
                 toolbar.failed();
-                Cryptpad.alert(Messages.disconnectAlert);
+                Cryptpad.alert(Messages.common_connectionLost);
             };
 
             var onConnectionChange = config.onConnectionChange = function (info) {
@@ -793,7 +793,7 @@ define([
                     toolbar.reconnecting(info.myId);
                     Cryptpad.findOKButton().click();
                 } else {
-                    Cryptpad.alert(Messages.disconnectAlert);
+                    Cryptpad.alert(Messages.common_connectionLost);
                 }
             };