Open the journald following descriptor earlier

Following a journal log almost always requires a descriptor to be
allocated.  In cases where we're running out of descriptors, this means
we might get stuck while attempting to start following the journal, at a
point where it's too late to report it to the client and clean up
easily.  The journal reading context will cache the value once it's
allocated, so here we move the check earlier, so that we can detect a
problem when we can still report it cleanly.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
(cherry picked from commit ab62ecf393b92d1e644f82c4711b6618b7c572a5)