GUACAMOLE-414: Convert errors to strings from ptread_mutex_lock and unlock.
GUACAMOLE-414: Convert errors to strings from ptread_mutex_lock and unlock.

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