transaction: raise on backup restoration error A few line above, similar errors in the truncation code result in raising the associated exception. We should do the same here. This means the transaction recover is more strict now, which might be a problem when running `hg recover` in a share different from the one where the transaction fails. However this has always been a problem and need to be be addressed independently.

transaction: add clarifying comment about why ignoring some error is fine It is less scary when explained.