Fix OpenStack module failure output

The exception message, when shade fails, will contain much more
specific information about the failure if the exception is treated
as a string. The 'message' attribute alone is usually not helpful.

lib/ansible/modules/cloud/openstack/os_server_volume.py

@@ -144,7 +144,7 @@ def main():
             )
 
     except (shade.OpenStackCloudException, shade.OpenStackCloudTimeout) as e:
-        module.fail_json(msg=e.message)
+        module.fail_json(msg=str(e))
 
 # this is magic, see lib/ansible/module_utils/common.py
 from ansible.module_utils.basic import *