Document what to do on non-matching plugin versions.

include/mosquitto_plugin.h

@@ -123,6 +123,9 @@ struct mosquitto_acl_msg {
  * return the plugin interface version you support, i.e. 5.
  *
  * The supported_versions array tells you which plugin versions the broker supports.
+ *
+ * If the broker does not support the version that you require, return -1 to
+ * indicate failure.
  */
 int mosquitto_plugin_version(int supported_version_count, const int *supported_versions);
 

src/security.c

@@ -370,8 +370,8 @@ static int security__module_init_single(struct mosquitto__listener *listener, st
 				}
 			}else{
 				log__printf(NULL, MOSQ_LOG_ERR,
-						"Error: Incorrect auth plugin version (got %d, expected %d).",
-						version, MOSQ_AUTH_PLUGIN_VERSION);
+						"Error: Unsupported auth plugin version (got %d, expected %d).",
+						version, MOSQ_PLUGIN_VERSION);
 				LIB_ERROR();
 
 				LIB_CLOSE(lib);