Unwind the stack on task failure

When a task fails, we will throw an exception, then catch it at the bottom of
the stack.

On Windows we don't do this yet because the exception doesn't propagate
correctly.

No cleanups yet.

Issue #236

src/rt/rust_task.h

@@ -155,6 +155,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
 
     // Fail self, assuming caller-on-stack is this task.
     void fail();
+    void conclude_failure();
 
     // Disconnect from our supervisor.
     void unsupervise();