docs/PBBP: Specify how to a transaction ends

References #6
docs/PBBP: Improve error reporting on the bus

The protocol is now more explicit about how to handle errors. In
particular:
 - When a slave sends a nack, it should be followed by another byte
   containing an error code.
 - The meaning of this error code can change depending on the current
   command.
 - After the error code, the slave drops off the bus and the master ends
   the transaction.
 - Trying to change a read-only EEPROM byte will now return a nack.

References #5
docs/PBBP: Improve spacing of table