added CTX_wm_operator_poll_msg_get/set so failing poll functions can set messages when poll fails, at the moment only python uses this but theres nothing python specific.

only added 1 message to a poll function, so messages still need to be set in many more places to be useful.

source/blender/blenkernel/BKE_context.h

@@ -163,6 +163,8 @@ void CTX_wm_screen_set(bContext *C, struct bScreen *screen); /* to be removed */
 void CTX_wm_area_set(bContext *C, struct ScrArea *sa);
 void CTX_wm_region_set(bContext *C, struct ARegion *region);
 void CTX_wm_menu_set(bContext *C, struct ARegion *menu);
+const char *CTX_wm_operator_poll_msg_get(struct bContext *C);
+void CTX_wm_operator_poll_msg_set(struct bContext *C, const char *msg);
 
 /* Data Context
 

source/blender/blenkernel/intern/context.c

@@ -64,6 +64,7 @@ struct bContext {
 		struct ARegion *region;
 		struct ARegion *menu;
 		struct bContextStore *store;
+		const char *operator_poll_msg; /* reason for poll failing */
 	} wm;
 	
 	/* data context */
@@ -399,6 +400,16 @@ void CTX_wm_menu_set(bContext *C, ARegion *menu)
 	C->wm.menu= menu;
 }
 
+void CTX_wm_operator_poll_msg_set(bContext *C, const char *msg)
+{
+	C->wm.operator_poll_msg= msg;
+}
+
+const char *CTX_wm_operator_poll_msg_get(bContext *C)
+{
+	return C->wm.operator_poll_msg;
+}
+
 /* data context utility functions */
 
 struct bContextDataResult {

source/blender/editors/screen/screen_ops.c

@@ -131,7 +131,11 @@ int ED_operator_view3d_active(bContext *C)
 
 int ED_operator_region_view3d_active(bContext *C)
 {
-	return CTX_wm_region_view3d(C) != NULL;
+	if(CTX_wm_region_view3d(C))
+		return TRUE;
+	
+	CTX_wm_operator_poll_msg_set(C, "expected a view3d region");
+	return FALSE;	
 }
 
 int ED_operator_timeline_active(bContext *C)

source/blender/editors/space_view3d/view3d_edit.c

@@ -1811,7 +1811,6 @@ static int view3d_zoom_border_invoke(bContext *C, wmOperator *op, wmEvent *event
 
 void VIEW3D_OT_zoom_border(wmOperatorType *ot)
 {
-
 	/* identifiers */
 	ot->name= "Border Zoom";
 	ot->description = "Zoom in the view to the nearest object contained in the border";

source/blender/python/intern/bpy_operator.c

@@ -127,7 +127,9 @@ static PyObject *pyop_call( PyObject * self, PyObject * args)
 	Py_XINCREF(context_dict); /* so we done loose it */
 
 	if(WM_operator_poll((bContext*)C, ot) == FALSE) {
-		PyErr_Format( PyExc_SystemError, "Operator bpy.ops.%.200s.poll() failed, context is incorrect", opname);
+		const char *msg= CTX_wm_operator_poll_msg_get(C);
+		PyErr_Format( PyExc_SystemError, "Operator bpy.ops.%.200s.poll() %s", opname, msg ? msg : "failed, context is incorrect");
+		CTX_wm_operator_poll_msg_set(C, NULL); /* better set to NULL else it could be used again */
 		error_val= -1;
 	}
 	else {

source/blender/windowmanager/intern/wm_event_system.c

@@ -466,6 +466,8 @@ static int wm_operator_exec(bContext *C, wmOperator *op, int repeat)
 	wmWindowManager *wm= CTX_wm_manager(C);
 	int retval= OPERATOR_CANCELLED;
 	
+	CTX_wm_operator_poll_msg_set(C, NULL);
+	
 	if(op==NULL || op->type==NULL)
 		return retval;
 	
@@ -698,6 +700,8 @@ static int wm_operator_call_internal(bContext *C, wmOperatorType *ot, int contex
 	
 	int retval;
 
+	CTX_wm_operator_poll_msg_set(C, NULL);
+
 	/* dummie test */
 	if(ot && C) {
 		switch(context) {