transaction: support cache file in backupentries We do not want to abort if anything wrong happen while handling a cache file. Cache file have way to be invalidated and if old/bad version stay no misbehavior will happen. Proper value will eventually be computed and the wrong will be righten. This changeset use the transaction reporter (usually writing on stderr) to write details about failed cache handling. This will only apply to write operation using a transaction. The usual update during read only operation will stay a debug message. I was on the way to bring these message back to debug level when I realised it could be a feature. People with write access to the repository are likely to have the power to fix error related to cache (and it is valuable to fix them). So let the things as is for now.