I2C Central Error Handling
When using UNOs you have direct controlability. With I2C you need a master and a bunch of slaves (you already know this). 

A good place to start is having the master ask the slaves periodically for an error report. If a slave sends an error or a slave does not respond the master shuts down the system. 

For an even more robust system (but much harder to set up) is that the connection is bidirectional. The master must hear from the slaves with no errors, if it does not it will shut down the system. The problem is that if the master fails the system can run out of control. If the slaves have to periodically also hear from the master to stay up then there is redundancy. In this way if the master fails the slaves will automatically shut down. This is hard to set up because setting up link and keep it stable can be problematic. 

My suggestion would be to bring the system up in disable mode. Then when the link is up and stable push a button on the master to enable the system.