lvm-dbus: Report error when function returns no object and no job
lvm-dbus: Report error when function returns no object and no job

For functions that return two object paths, one of them must be
a valid path -- either the function returns a newly created object
or a valid job, if it return "/", "/" we should report error.

rhinstaller/libblockdev,rhinstaller/libblockdev,rhinstaller/libblockdev