Revert Graph if commit fails

[white-gecko]

If the write operation of the commit fails, we need to revert the graph in the store (in memory) as well.

QuitStore/quit/core.py

Lines 426 to 434 in 76f798b

	resultingChanges, exception = graph.update(parsedQuery)
	if exception:
	# TODO need to revert or invalidate the graph at this point.
	pass
	oid = self.commit(graph, resultingChanges, 'New Commit from QuitStore', parent_commit_ref,
	target_ref, query=query, default_graph=default_graph,
	named_graph=named_graph)
	if exception:
	raise exception

There needs to be a try … except block around the commit method, which would revert the update operation/invalidate the graph in case of an exception.

Somehow related to #191