I know what went wrong: sometimes Blender (or PyPRP) will loose track of a texture, which is why you get this message.
To fix it you simply have to note which object causes the problem (it is displayed in the console just above the 'Traceback most recent call' part) then select that object, make sure you have a UV/Image editor window opened, and hit 'F'. That will display the 'missing' texture in the Image editor window and that will fix your problem.
It happens from time to time, I have no idea what is the exact source of this, but at least it's easy to fix.