GUACAMOLE-986: Add non-nullable modifier to return type of functions never returning null

Alternatively use a primitive type instead of nullable boxed type.

guacamole-common-js/src/main/webapp/modules/Status.js

@@ -58,7 +58,7 @@ Guacamole.Status = function(code, message) {
 
     /**
      * Returns whether this status represents an error.
-     * @returns {Boolean} true if this status represents an error, false
+     * @returns {boolean} true if this status represents an error, false
      *                    otherwise.
      */
     this.isError = function() {