Move the responsibility for process failure from tasks to the scheduler

When the root task fails the process fails. Failures on other tasks propagate
up the task tree. Failures on non-root tasks without parents just
(theoretically) unwind and disappear.

src/rt/rust_task.cpp

@@ -220,8 +220,6 @@ rust_task::fail() {
     backtrace();
     // Unblock the task so it can unwind.
     unblock();
-    if (this == sched->root_task)
-        sched->fail();
     if (supervisor) {
         DLOG(sched, task,
              "task %s @0x%" PRIxPTR
@@ -230,7 +228,8 @@ rust_task::fail() {
         supervisor->kill();
     }
     // FIXME: implement unwinding again.
-    exit(1);
+    if (this == sched->root_task)
+        sched->fail();
 }
 
 void