If a session load fails, subsequent calls to
session.load() would trigger another request immediately.
Together with feature flag checks triggering session.load(),
this could result in a cycle if no network connection was present:

 1. App loads during initial digest cycle, 'flag.featureEnabled()'
    is called to test whether to show certain UI elements
 2. session.load() is triggered
 3. session.load() fails and the HTTP response error triggers
    a digest cycle
 4. GOTO 1

This commit resolves the problem by automatically retrying
session.load() calls with an exponential backoff.