Assignment upgrade exception

Eloy reported an assignment upgrade exception with a null exception error message. "Could not convert assignment. Exception was: {$a}."

Extra info: object(comment_exception)[4892]
public 'errorcode' => string 'nopermissiontocomment' (length=21)
public 'module' => string 'error' (length=5)
public 'a' => null
public 'link' => string '' (length=0)
public 'debuginfo' => null
protected 'message' => string 'You can't add comments' (length=22)
private 'string' (Exception) => string '' (length=0)
protected 'code' => int 0
protected 'file' => string '/Users/stronk7/Sites/fp/moodle/comment/lib.php' (length=46)
protected 'line' => int 613
private 'trace' (Exception) =>

So 2 things need fixing: Assignment upgrade code needs to handle a null exception->error and more testing on upload assignments with comments re: permissions to try and reproduce this bug.