Modify upcall_fail so that failing tasks relinquish control to the scheduler

This is sufficient to let unsupervised tasks that don't allocate fail without
crashing or leaking.

Author: brson

src/rt/rust_upcall.cpp

@@ -221,6 +221,9 @@ upcall_fail(rust_task *task,
     LOG_UPCALL_ENTRY(task);
     LOG_ERR(task, upcall, "upcall fail '%s', %s:%" PRIdPTR, expr, file, line);
     task->fail();
+    task->die();
+    task->notify_tasks_waiting_to_join();
+    task->yield(4);
 }
 
 /**