Add getException to MatchResult
Tag MatchResult with type
Split listener into generic + multi-method handler
DiffListener - low level for raw events; DiffHandler - strong-typed one
Implement DiffResultsSummarizer
Summing up children match events into overall status for parent
DiffResultsSummarizer: support for missing/surplus
DataRowDiff: refactor init of children listeners
DataRowDiffTest - ensure result is not determined by last cell only
MatchResultTest - test setException changes status
Refactor DataTableDiff - using summarizer