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.
I think the metadata would not change after the reload... That's why I created #808 in the first place (together with the reason that tmp files might not get removed) - the files should be checked if they match or not (if they do not, this means something happened while copying the file from the DFS to the local FS), but the message should only be logged after `$this->maxCopyTries` has been reached, with the file sizes of the last try.

Here, we just continue the loop without copy try to be executed again, and the filesize values will thus not change.