Macro try_rethrow

macro_rules! try_rethrow {
    ($res:expr) => { ... };
}

Like try_throw!, but designed for TraceResults, as it keeps the previous trace.

This macro will try to call Trace::convert on the trace to convert the inner error if necessary, similarly to try!

This relies on the return type of the function to provide type inference for the Result::Ok(T) type.