crash: Fix session manager D-Bus call service name

The D-Bus call to session manager was using the incorrect service
name field, causing the crash reporter session lookup to fail and
fallback to placing the crash in /home/chronos/crash.  Fix the
D-Bus call so that crashes are put in the right place.

Also, fixing this exposed an error with processing the results.
The previous D-Bus code treated an empty session response the
same as a D-Bus connection error, with the new code D-Bus errors
are handled separately, so also check whether the returned array
is empty.

BUG=chromium:458330
TEST=triggered crash with 'chrome://crash' page; crash files are
     placed in /home/chronos/user/crash
TEST=`test_that -b panther <IP> logging_UserCrash` passes

Change-Id: I77733adc5e4287d9eae83a749818d9cc38398c80
Reviewed-on: https://chromium-review.googlesource.com/249800


Reviewed-by: Dan Erat <derat@chromium.org>
Tested-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>