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.
problem is, with concurrent processes, small files (expiry.php comes to mind) can change between two reads.   it's that one in a million fail that high stressed servers suffer. the 99.999% rest of the time, that do...while loop in the mysqli (hunk below)  will pass on first run.