Add verbose error messages at __virtual__

Change-Id: Ieb1a1ac405b5faac0e7bb9ce44df2f4869c5736c
diff --git a/_states/jenkins_job.py b/_states/jenkins_job.py
index 69deb37..74be81c 100644
--- a/_states/jenkins_job.py
+++ b/_states/jenkins_job.py
@@ -15,6 +15,18 @@
 log = logging.getLogger(__name__)
 
 
+def __virtual__():
+    '''
+    Only load if jenkins_common module exist.
+    '''
+    if 'jenkins_common.call_groovy_script' not in __salt__:
+        return (
+            False,
+            'The jenkins_job state module cannot be loaded: '
+            'jenkins_common not found')
+    return True
+
+
 def _elements_equal(e1, e2):
     if e1.tag != e2.tag:
         return False