Fail EC2 inventory better with restricted EC2 users

If a user has full EC2 access but no RDS access, the user
should still be able to query the EC2 inventory, although
will not obtain any information about any RDS instances.

plugins/inventory/ec2.py

@@ -295,9 +295,10 @@ class Ec2Inventory(object):
                 for instance in instances:
                     self.add_rds_instance(instance, region)
         except boto.exception.BotoServerError as e:
-            print "Looks like AWS RDS is down: "
-            print e
-            sys.exit(1)
+            if not e.reason == "Forbidden":
+                print "Looks like AWS RDS is down: "
+                print e
+                sys.exit(1)
 
     def get_instance(self, region, instance_id):
         ''' Gets details about a specific instance '''