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.
Yes, the metadata *should* not change, and neither should the dfs file size.
However, this does not appear to be the case (heavy concurrency, cache updates, etc); keep in mind there may be multiple requests/operations being executed almost simultaneously.
As all things DFS, trying to reproduce it on a test environment under controlled conditions may not be fun...