Fix EZP-23092: incorrect error message when fetching from DFS

JIRA: https://jira.ez.no/browse/EZP-23092

If there is a mismatch between metadata size and dfs size, the error message generated by dfs fetch will be incoherent:
``'Size (100) of written data for file 'var/.../....cache123456tmp.php' does not match expected size 100'``

This fixes the message's second argument to the actual variable being checked, but also adds an additional validation of the metadata vs dfsfile size at each loop iteration.
Agree that logging should not be done until copy fails. Or at least it should be logged as warning.

As for files changing in-between loop passes: I think we should reload the size from both disk and db. And make sure clearstatcache() is called when reloading from disk. In other words: we allow the fetch to happen only when we get at least hold of a good situation, where db and nfs are in sync, even if that means waiting a little