Annotate the exception with the possible failure reason when Bitmaps are not enabled.
When bitmaps are not enabled the RevWalk is expensive. AllowFilter
provides an override to continue walking. PedestrianWalk can be
expensive and fails when the clone is a partial clone using
--filter=tree:0. This can only work when bitmaps are enabled.
Having a message to the exception improves debuggability of such cases.