Use ErrorList in estimators

Support it in both the abs + abs, and pot + abs and just abs.

Change-Id: I1c11d5d1fdc3d644f9d391bde7c06b6c6e8ec57d
Signed-off-by: Ravago Jones <ravagojones@gmail.com>
diff --git a/frc971/zeroing/absolute_encoder.h b/frc971/zeroing/absolute_encoder.h
index 0021e13..df40ec3 100644
--- a/frc971/zeroing/absolute_encoder.h
+++ b/frc971/zeroing/absolute_encoder.h
@@ -85,6 +85,9 @@
 
   // The filtered position.
   double position_ = 0.0;
+
+  // Marker to track what kind of error has occured.
+  aos::ErrorList<ZeroingError> errors_;
 };
 
 }  // namespace zeroing