selfcheck.py: make selfcheck work if there is no systemd module

clever/src/selfcheck.py

@@ -16,7 +16,6 @@ import traceback
 from threading import Event
 import numpy
 import rospy
-from systemd import journal
 import tf2_ros
 import tf2_geometry_msgs
 from pymavlink import mavutil
@@ -620,6 +619,12 @@ def check_clever_service():
         failure('clever.service is not running, try sudo systemctl restart clever')
         return
 
+    try:
+        from systemd import journal
+    except ImportError:
+        failure('no python-systemd package, not the Clever image?')
+        return
+
     j = journal.Reader()
     j.this_boot()
     j.add_match(_SYSTEMD_UNIT='clever.service')