Loader: reflect faults to client

With this patch, the loader installs an optional client-provided fault
handler as default CPU exception handler and RM fault handler for all
CPU and RM sessions of the loaded subsystem. This way, loader clients
become able to respond to failures occuring within the subsystem.

The new feature is provided via the added 'Loader::fault_handler' RPC
function.

The 'run/failsafe' test covers two cases related to the loader, which
are faults produced by the immediate child of the loader and faults
produced by indirect children.
Add failsafe test to autopilot