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.
Actually, that is exactly the problem.
It seems `$dfsFileSize` and `$metadata['size']` may not match at some point, and there is no guarantee they will after getting the size again (even though they should obviously be the same).
Perhaps metadata should also be reloaded at this stage?