Add missing dbus_error_init() in UNKNOWN_OBJECT case. Fixes assertion

systemd[1]: Caught <ABRT>, dumped core as pid 6256.
systemd[1]: Freezing execution.

Core was generated by `/bin/systemd systemd.unit=graphical.target'.
Program terminated with signal 6, Aborted.
#0  0x00007f8966ec81db in raise () from /lib64/libpthread.so.0
(gdb) bt
#0  0x00007f8966ec81db in raise () from /lib64/libpthread.so.0
#1  0x000000000040823b in crash (sig=6) at src/main.c:120
#2  <signal handler called>
#3  0x00007f896613c075 in raise () from /lib64/libc.so.6
#4  0x00007f896613d806 in abort () from /lib64/libc.so.6
#5  0x00007f89672ac8a5 in _dbus_abort () at dbus-sysdeps.c:94
#6  0x00007f89672a37b5 in _dbus_warn_check_failed (
   format=0x7f89672b35d8 "arguments to %s() were incorrect, assertion
\"%s\" failed in file %s line %d.\nThis is normally a bug in some
application using the D-Bus library.\n") at dbus-internals.c:289
#7  0x0000000000441500 in bus_unit_message_handler (connection=0x21b6090,
   message=0x21b6760, data=0x1f3e870) at src/dbus-unit.c:572
#8  0x00007f8967299f11 in _dbus_object_tree_dispatch_and_unlock (
   tree=0x219d660, message=0x21b6760) at dbus-object-tree.c:858
#9  0x00007f896728bca2 in dbus_connection_dispatch (connection=0x21b6090)
   at dbus-connection.c:4688
#10 0x000000000043befa in bus_dispatch (m=0x1f3e870) at src/dbus.c:547
#11 0x000000000041056d in manager_loop (m=0x1f3e870) at src/manager.c:2344
#12 0x0000000000409515 in main (argc=<value optimized out>,
   argv=<value optimized out>) at src/main.c:1229