Failure: Include message and backtrace from previous exceptions.

If an exception wraps another, it is impossible to tell what the cause
was looking at the outer exception only. With this commit, when a job
fails the full backtrace of all previous exceptions is recorded.

Example:

    Cannot create User.
    #0 file.php(1): createUser

    Caused by: Cannot connect to database.
    #0 db.php(1): connect