No 'server connection lost' message with long-polling push

Originally by mpilone

When using long-polling push with no polling configured in the UI, the 'server connection lost' message is not displayed on the client side when the connection to the server is dropped.

This was tested using an application that only configures push and does not set a polling interval in the UI. The client machine was then disconnected from the network (removed the network cable). The client was not alerted that the connection to the server is down (the message in the upper right by default). If a polling interval is enabled, the client does get the message after a failed polling attempt.

It is clear that the client side push connection gets dropped and it tries to reconnect repeatedly so it seems like it has the information to know that the server connection was dropped. For long running push applications, it would be very useful to display when the connection is dropped so the client is aware that it will no longer get updates from the server until it resolves the issue.

This is similar to my other issue https://dev.vaadin.com/ticket/19822 where polling must be enabled as a workaround even for an all push driven application. The polling interval must be small enough to trigger the connection notification even if polling isn't desired.

Imported from https://dev.vaadin.com/ issue #19841

No source file or test is named in the issue. Start by locating the client-side long-polling push reconnect and error handling, then reproduce with push enabled and polling disabled while disconnecting the client. Done means the server-connection-lost message appears after the connection drops.