Yes, you would get a ClassNotFoundException if you tried to set a formatter that doesn't exist on the Accumulo classpath. Additionally, if there was one previously set on the table, you would, I think, see the same ClassNotFoundException when you try to switch to that table in the shell. This could very easily happen if someone upgrades Accumulo and forgets to copy the jars from the old Accumulo installation's lib/.
Is there a desired benefit to falling back onto the DefaultFormatter if the specified formatter cannot be loaded (ClassNotFoundException, ClassCastException, etc)? Having a missing formatter class set will mess up all the scans until removed or the classpath being fixed; however, (Batch)Scanners won't be affected.