Hint when PHP upload size is not enough

Thanks for the plugin!

I ran into this issue: I wanted to upload a template kit but got the information that the template format is invalid. However, it turned out that the PHP’s upload_max_filesize was not enough.

I guess a more meaningful message could be returned in class-template-kit-install.php when $all_files['file']['error'] is checked, i.e. $all_files['file']['error'] == 1 (UPLOAD_ERR_INI_SIZE) indicates this scenario.