separate ability and capability

These are asking fundamentally different questions;

Abilities are asking the application if the user has a role that allows
the user to take a certain action
Capabilities are asking if the user has granted the application to
perform a certain type of action

CanCanCan makes no distinction, however, so the `granted_capabilities`
method is provided as a point that can be checked in rescue methods, so
that one can _attempt_ to continue to provide the more informative error
messages around permission refusals