Drivers: implement fallback values for RNA path based variables.

As discussed in #105407, it can be useful to support returning
a fallback value specified by the user instead of failing the driver
if a driver variable cannot resolve its RNA path. This especially
applies to context variables referencing custom properties, since
when the object with the driver is linked into another scene, the
custom property can easily not exist there.

This patch adds an optional fallback value setting to properties
based on RNA path (including ordinary Single Property variables
due to shared code and similarity). When enabled, RNA path lookup
failures (including invalid array index) cause the fallback value
to be used instead of marking the driver invalid.

A flag is added to track when this happens for UI use. It is
also exposed to python for lint type scripts.

When the fallback value is used, the input field containing
the property RNA path that failed to resolve is highlighted in red
(identically to the case without a fallback), and the driver
can be included in the With Errors filter of the Drivers editor.
However, the channel name is not underlined in red, because
the driver as a whole evaluates successfully.

Pull Request: https://projects.blender.org/blender/blender/pulls/110135

scripts/startup/bl_ui/space_graph.py

@@ -93,6 +93,7 @@ class GRAPH_PT_filters(DopesheetFilterPopoverBase, Panel):
 
     def draw(self, context):
         layout = self.layout
+        st = context.space_data
 
         DopesheetFilterPopoverBase.draw_generic_filters(context, layout)
         layout.separator()
@@ -100,6 +101,12 @@ class GRAPH_PT_filters(DopesheetFilterPopoverBase, Panel):
         layout.separator()
         DopesheetFilterPopoverBase.draw_standard_filters(context, layout)
 
+        if st.mode == 'DRIVERS':
+            layout.separator()
+            col = layout.column(align=True)
+            col.label(text="Drivers:")
+            col.prop(st.dopesheet, "show_driver_fallback_as_error")
+
 
 class GRAPH_PT_snapping(Panel):
     bl_space_type = 'GRAPH_EDITOR'