Have Chunked upload jobs bail if cannot associate with session.

Also add docs about how suhosin.session.encrypt tends to break
this (I was thinking about throwing an exception if the setting was
on in RequestContext::importScopedSession, but the docs seem to
indicate that you can tweak how the session is encrypted, so it
sounds like somebody could set up their php so everything worked).

Bug: 48371
Change-Id: I5a471c1f941fce046451fbb9abce1c830185cabb
Merge "Have Chunked upload jobs bail if cannot associate with session."