Re-wording error budgets

I am rewording error budgets to encourage risk management, rather than conservatism.

I am also reducing the budget from 30 to 15 due to this since a 30 point budget would unintentionally encourage a production S1 incident.

Note that Manage and CI already have an S2 apiece. So those teams shoudl manage their risk better, and get assistance from SREs to do so.

@glopezfernandez @dhavens @timzallmann @tommy.morgan please communicate the change to your teams