When uploading a mbz backup file that exceeds the php upload limit the error message shown is "A required parameter (sesskey) was missing".

It should read something like "upload failed due to file size limit. See https://docs.moodle.org/en/File_upload_size".

Tested on moodle versions 2.7.2 (Build: 20140908) and 2.8rc1 (Build: 20141104)

apache error log:
PHP Warning: POST Content-Length of 218634755 bytes exceeds the limit of 8388608 bytes in Unknown on line 0, referer: http://moodle.lo/backup/restorefile.php?contextid=2