kiji-schema: SCHEMA-687. DebugResourceTracker cannot accept null error messages.

issue: https://jira.kiji.org/browse/SCHEMA-687
review: https://review.kiji.org/r/1746
commit: https://github.com/kijiproject/kiji-schema/commit/8bac2698fb66b201fcde039f86b2a820e8ac19a1