Try to fail nicely, again.

src/rt/rust_task.cpp

@@ -505,6 +505,8 @@ rust_task::fail(size_t nargs) {
              name, this, supervisor->name, supervisor);
         supervisor->kill();
     }
+    // FIXME: implement unwinding again.
+    exit(1);
 }
 
 void

src/rt/rust_upcall.cpp

@@ -275,8 +275,6 @@ upcall_fail(rust_task *task,
     LOG_UPCALL_ENTRY(task);
     LOG_ERR(task, upcall, "upcall fail '%s', %s:%" PRIdPTR, expr, file, line);
     task->fail(4);
-    // FIXME: re-implement unwinding.
-    abort();
 }
 
 /**