Sorry that this took a while, the holidays got in the way. ;-)
Anyway, extremely clean code, very well done. I second David -- I don't know much about this payment processor either.
While I see the advantage of putting all Exceptions into the same file, I also think this breaks the class<->file layout, so I'm torn on that. Maybe the PEAR overlords can comment if this is "allowed".
I have some minor edits, so I forked your repo on Github -- have a look, the changes are not conditional, just food for thought!