[Caching] Prevent IOE from happening in TreeNode::ResetCacheData()

If AddinDatabase.ResetCacheData() ends up calling TreeNode.ResetCacheData(),
an InvalidOperationException could be thrown when trying to read an addin
description, because the "basic cache data" has already been reset by
AddinDatabase.ResetBasicCacheData(). In this case, just bail and ignore.

Fixes http://monoaddins.codeplex.com/workitem/8364