GUACAMOLE-414: Use correct formatting for string from strerror.
GUACAMOLE-414: Use correct formatting for string from strerror.

apache/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,glyptodon/guacamole-server