Use a unique exit status when the runtime fails normally

Check for it in run-fail tests

src/rt/rust_scheduler.cpp

@@ -71,7 +71,7 @@ rust_scheduler::fail() {
     log(NULL, log_err, "domain %s @0x%" PRIxPTR " root task failed",
         name, this);
     I(this, kernel->rval == 0);
-    kernel->rval = 1;
+    kernel->rval = PROC_FAIL_CODE;
     kernel->fail();
 }