[ticket/11467] Add language variables for extension exception messages

Remove hard-coded messages and add language variables for extension
exception messages.

PHPBB3-11467

phpBB/language/en/acp/extensions.php

@@ -117,4 +117,7 @@ $lang = array_merge($lang, array(
 	'VERSIONCHECK_FORCE_UPDATE_ALL'		=> 'Re-Check all versions',
 	'FORCE_UNSTABLE'					=> 'Always check for unstable versions',
 	'EXTENSIONS_VERSION_CHECK_SETTINGS'	=> 'Version check settings',
+
+	'META_FIELD_NOT_SET'	=> 'Required meta field %s has not been set.',
+	'META_FIELD_INVALID'	=> 'Meta field %s is invalid.',
 ));

phpBB/language/en/common.php

@@ -231,6 +231,8 @@ $lang = array_merge($lang, array(
 	'FILESIZE'				=> 'File size',
 	'FILEDATE'				=> 'File date',
 	'FILE_COMMENT'			=> 'File comment',
+	'FILE_CONTENT_ERR'		=> 'Could not read the contents of file: %s',
+	'FILE_JSON_DECODE_ERR'	=> 'Failed to decode json file: %s',
 	'FILE_NOT_FOUND'		=> 'The requested file could not be found: %s',
 	'FIND_USERNAME'			=> 'Find a member',
 	'FOLDER'				=> 'Folder',