rt: Shutdown gracefully on failure

When the kernel fails, kill all tasks and wait for the schedulers to stop
instead of just exiting. I'm sure there are tons of lurking issues here but
this is enough to fail without leaking (at least in the absence of cleanups).

src/rt/rust_kernel.cpp

@@ -134,6 +134,14 @@ int rust_kernel::start_task_threads()
     return rval;
 }
 
+void
+rust_kernel::fail() {
+    for(size_t i = 0; i < num_threads; ++i) {
+        rust_scheduler *thread = threads[i];
+        thread->kill_all_tasks();
+    }
+}
+
 rust_task_id
 rust_kernel::create_task(rust_task *spawner, const char *name) {
     rust_scheduler *thread = threads[rand(&rctx) % num_threads];