Refactor: scheduler: make unpacking resource failures more efficient

Previously, for some failure results, unpack_rsc_op() created a transient
instance of an action using custom_action() (via get_action_on_fail() and
a second time via unpack_rsc_op_failure() if called).

However, it only needed the action's configured on-fail and role after failure.
Now, use the new parsing functions to get just those, allowing us to avoid the
rest of custom_action()'s processing.

This involves broadening get_action_on_fail() to unpack_failure_handling() and
moving it up in the file.