Add verbose error messages at __virtual__

Change-Id: Ieb1a1ac405b5faac0e7bb9ce44df2f4869c5736c
diff --git a/_states/jenkins_plugin.py b/_states/jenkins_plugin.py
index 4956751..cb54633 100644
--- a/_states/jenkins_plugin.py
+++ b/_states/jenkins_plugin.py
@@ -77,6 +77,18 @@
 """ # noqa
 
 
+def __virtual__():
+    '''
+    Only load if jenkins_common module exist.
+    '''
+    if 'jenkins_common.call_groovy_script' not in __salt__:
+        return (
+            False,
+            'The jenkins_plugin state module cannot be loaded: '
+            'jenkins_common not found')
+    return True
+
+
 def present(name, restart=False):
     """
     Jenkins plugin present state method, for installing plugins