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.
@pbras is right, @andrerom. statcache does need to be cleared when reading due to concurrency.