Add verbose error messages at __virtual__

Change-Id: Ieb1a1ac405b5faac0e7bb9ce44df2f4869c5736c
diff --git a/_states/jenkins_slack.py b/_states/jenkins_slack.py
index 73048fe..cbe52ec 100644
--- a/_states/jenkins_slack.py
+++ b/_states/jenkins_slack.py
@@ -25,6 +25,19 @@
 }}
 """ # noqa
 
+
+def __virtual__():
+    '''
+    Only load if jenkins_common module exist.
+    '''
+    if 'jenkins_common.call_groovy_script' not in __salt__:
+        return (
+            False,
+            'The jenkins_slack state module cannot be loaded: '
+            'jenkins_common not found')
+    return True
+
+
 def config(name, team_domain, token, token_credential_id="", room="", send_as=None):
     """
     Jenkins Slack config state method