Macro trace_error::try_throw

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

Like try!, but invokes throw! on the error value if it exists, converting it to Result::Err(Trace<E>)

Note that the backtrace will only go as far as the location this macro was invoked

This macro will try to call From::from on the error to convert it if necessary, just like try!

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