flush tw_output calls on tw_error?

[JohnPJenkins]

While reviewing #75 I thought of the following - tw_suspend may or may not cause a tw_error down the line. Since tw_suspend doesn't take output arguments (only an errno equivalent), tw_output would be a good alternative as it is rollback-aware - it only gets printed out if the suspend persists through the rest of the simulation. However, if tw_error is called before the output buffer is flushed (ie through short-circuiting the simulation), then the output is lost.

Can we flush tw_output when erroring out?