upgrade 2.1 to 2.3

I started the upgrade after copying over the files and received this message.

I can not longer log into the original gallery and am in the process of reverting from the back up.
However. What does this mean?

Notice: Use of undefined constant GALLERY_FORM_VARIABLE_PREFIX - assumed 'GALLERY_FORM_VARIABLE_PREFIX' in /home/content/C/h/u/ChuckandMarie/html/gallery/modules/core/classes/GalleryUtilities.class on line 242

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/content/C/h/u/ChuckandMarie/html/gallery/modules/core/classes/GalleryUtilities.class:242) in /home/content/C/h/u/ChuckandMarie/html/gallery/lib/support/GallerySetupUtilities.class on line 75

Did a similar upgrade on another site and it went smoothly