Save errors for use when encountering "Left over data" error.

When people get "left over data" Daffodil has already discarded the errors causing backtracking that may have made the parse stop short on the data.

We should save the errors (not all, but the most recent ones) and make them available for help debugging this situation.

See email thread https://lists.apache.org/thread/68ydpm09lco4yl0v2pt2np62lftgzx1t