Drivers Code Cleanups and UI Tweaks:

- Adding drivers from the UI (not from py-scripts though) will now
automatically add a "Transform Channel" driver variable to the newly
created drivers. This makes setting up drivers a bit more convenient
for the most commonly used case.

- Drivers now report their errors using the Reports system instead of
writing these directly to the console.

- Clarified some comments to be more insightful about the "why's" of
some design decisions, and related formatting/cleanup tweaks
- Reduced scope of "path" vars to just the scope they're required in

- Removed some unused defines from a failed experiment in the original
Keying Sets code ("templates" and "template flags") which was
superseeded by the more flexible + nicer "Builtin KeyingSets"

source/blender/python/intern/bpy_rna.c

@@ -2241,7 +2241,7 @@ static PyObject *pyrna_struct_driver_add(BPy_StructRNA *self, PyObject *args)
 	if(pyrna_struct_anim_args_parse(&self->ptr, "bpy_struct.driver_add():", path,  &path_full, &index) < 0) 
 		return NULL;
 
-	if(ANIM_add_driver((ID *)self->ptr.id.data, path_full, index, 0, DRIVER_TYPE_PYTHON)) {
+	if(ANIM_add_driver(/*ReportList*/NULL, (ID *)self->ptr.id.data, path_full, index, 0, DRIVER_TYPE_PYTHON)) {
 		ID *id= self->ptr.id.data;
 		AnimData *adt= BKE_animdata_from_id(id);
 		FCurve *fcu;
@@ -2300,7 +2300,7 @@ static PyObject *pyrna_struct_driver_remove(BPy_StructRNA *self, PyObject *args)
 	if(pyrna_struct_anim_args_parse(&self->ptr, "bpy_struct.driver_remove():", path,  &path_full, &index) < 0) 
 		return NULL;
 
-	ret= PyBool_FromLong(ANIM_remove_driver((ID *)self->ptr.id.data, path_full, index, 0));
+	ret= PyBool_FromLong(ANIM_remove_driver(/*ReportList*/NULL, (ID *)self->ptr.id.data, path_full, index, 0));
 
 	MEM_freeN(path_full);