Export fetchRootConfig status so it can be read by TK (#14053)

Offline apps need to know when server errors are 500 or 400 in
order to switch appropriatelly to the offline mode.
Also we need exported the fetchRootConfig method and a
reliable way to get loaded apps.

Related with change I29635982514071e63221a9771d6729da14273ad3 [1]
see temporal workaround in TouchKitServlet
[1] https://dev.vaadin.com/review/#/c/4037/

Change-Id: I339ca697d035508a67d1eb24480cd12c4b9c6c0e