radiusproxy plugin: Use str(error) rather than error.message

In Python 3, the "message" attribute has been removed in favor of
calling str() on the error.

Part of the work for https://fedorahosted.org/freeipa/ticket/4985

Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Petr Spacek <pspacek@redhat.com>

ipalib/plugins/radiusproxy.py

@@ -85,7 +85,7 @@ def validate_radiusserver(ugettext, server):
         validate_hostname(server, check_fqdn=True, allow_underscore=True)
     except ValueError as e:
         raise errors.ValidationError(name="ipatokenradiusserver",
-                                     error=e.message)
+                                     error=str(e))
 
 
 @register()