Detect user navigating away, don't interpret as ajax error

Whenever a user navigates away, all ajax calls will fail with the same
result like a cross-domain redirect (SSO). To distinguish these cases,
we need to detect whether the error is a result of the user navigating
away. For this, we introduce a new flag that will be set in
"beforeunload".

Additional handling was required for false positives in case "beforeunload" is
used (ex: cancelled upload) and the user cancelled the navigation.