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. If this pull request is merged, when a job fails the full backtrace of all previous exceptions is recorded.

Before:

    Cannot create User.
    #0 file.php(1): createUser

After:

    Cannot create User.
    #0 file.php(1): createUser

    Caused by: Cannot connect to database.
    #0 db.php(1): connect