Don't abort the process in native::start

If the main closure failed, then the `exit_code` variable would still be `None`,
and the `unwrap()` was failing (triggering a process abort). This changes the
`unwrap()` to an `unwrap_or()` in order to prevent process abort and detect when
the native task failed.

src/libnative/lib.rs

@@ -73,7 +73,8 @@ pub fn start(argc: int, argv: **u8, main: proc()) -> int {
         exit_code = Some(run(main.take_unwrap()));
     });
     unsafe { rt::cleanup(); }
-    return exit_code.unwrap();
+    // If the exit code wasn't set, then the task block must have failed.
+    return exit_code.unwrap_or(rt::DEFAULT_ERROR_CODE);
 }
 
 /// Executes a procedure on the current thread in a Rust task context.

src/test/run-fail/native-failure.rs

@@ -0,0 +1,22 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern:explicit failure
+
+#[no_uv];
+
+extern mod native;
+
+#[start]
+fn start(argc: int, argv: **u8) -> int {
+    do native::start(argc, argv) {
+        fail!();
+    }
+}