mode: reset mode on entry to fn body.

This is an interim fix to address the "Beware!" unsoundness.  I have a
more comprehensive rewrite of mode.rs in the pipeline.

r=pcwalton

src/libcore/condition.rs

@@ -37,7 +37,7 @@ impl<T, U> Condition<T, U> {
 
     fn raise(t: T) -> U {
         let msg = fmt!("Unhandled condition: %s: %?", self.name, t);
-        self.raise_default(t, || fail msg)
+        self.raise_default(t, || fail copy msg)
     }
 
     fn raise_default(t: T, default: &fn() -> U) -> U {