Make TclBackgroundException() Public

When background operations in Tcl result in a non-TCL_OK return code, it has been recommended practice to call Tcl_BackgroundError() so that Tcl can arrange for background error handlers to react to the exceptional situation. The prototype of that routine is: void Tcl_BackgroundError(Tcl_Interp *interp) In Tcl 8.5, the new interp bgerror command permits registration of handlers that receive the full return options dictionary as an argument . This means that such handlers can respond differently to different exceptions. At the same time a new internal routine TclBackgroundException with prototype: void TclBackgroundException(Tcl_Interp *interp, int code) and the implementation of Tcl_BackgroundError became: dm9pZCBUY2xfQmFja2dyb3VuZEVycm9yKFRjbF9JbnRlcnAgKmludGVycCkgew==ICAgIFRjbEJhY2tncm91bmRFeGNlcHRpb24oaW50ZXJwLCBUQ0xfRVJST1IpOw==fQ== The new TclBackgroundException routine is a more general routine suitable for announcing any exceptional code returned by background operations, not limited to TCL_ERROR like Tcl_BackgroundError has been. The new TclBackgroundException routine is used throughout the internal portions of Tcl so that full exception information gets reliably passed through to background exception handlers, when the background processing is governed by things which are built in to Tcl, such as after and chan event. The oversight of was not proposing to make TclBackgroundException public at the same time, so that extensions which govern background operations, notably Tk, have access to the same full expressive power.