Permission: Throw exception on unknown values in getByName()

Previously, writing wrong values into plugin.yml for permission defaults would cause the permission to be silently denied to everyone.

src/pocketmine/permission/Permission.php

@@ -42,6 +42,8 @@ class Permission{
 	 * @param bool|string $value
 	 *
 	 * @return string
+	 *
+	 * @throws \InvalidArgumentException
 	 */
 	public static function getByName($value) : string{
 		if(is_bool($value)){
@@ -70,10 +72,11 @@ class Permission{
 
 			case "true":
 				return self::DEFAULT_TRUE;
-
-			default:
+			case "false":
 				return self::DEFAULT_FALSE;
 		}
+
+		throw new \InvalidArgumentException("Unknown permission default name \"$value\"");
 	}
 
 	/**