Error messages are not verbose
If swiftmailer can't read a file in the function _getReadHandle in typo3/contrib/swiftmailer/classes/Swift/ByteStream/FileByteStream.php it will present this error message to the user:
"Unable to open file for reading [FILENAME]"
If you have turned allow_url_fopen OFF PHP returns such errors:
Warning: fopen() [function.fopen]: URL file-access is disabled in the server configuration in XYZ on line 3
so the PHP error message is more verbose and tells the user exactely where he should look. TYPO3 could generally check the environment and warn the user that certain features won't work without the necessary PHP.INI settings.
see also postings: