cli: Use g_warning() instead of g_print() for error

Output before:

    Path                                                                                                                                           Recursive
    /home/pi/Desktop                                                                                                                               *
    Could not expand XDG user directory &DOCUMENTSCould not expand XDG user directory &MUSICCould not expand XDG user directory &PICTURESCould not expand XDG user directory &VIDEOS/home/pi                                                                                                                                       -

Output after:

    Path                                                                                                                                           Recursive
    /home/pi/Desktop                                                                                                                               *

    (tracker index:28168): Tracker-WARNING **: 14:41:09.497: Could not expand XDG user directory &DOCUMENTS

    (tracker index:28168): Tracker-WARNING **: 14:41:09.498: Could not expand XDG user directory &MUSIC

...
4 jobs for sam/tracker-index-gwarning in 8 minutes and 34 seconds
Status Job ID Name Coverage
  Test
passed #1349377
test-fedora-latest

00:01:21

passed #1349378
test-fedora-rawhide

00:01:55

passed #1349379
test-ubuntu-rolling

00:01:32

 
  Analysis
passed #1349380
coverage-analysis

00:06:38

49.3%