global: refextract calls surrounded in try blocks

* Sometimes the refextract calls produce a RuntimeError
  maximum recursion depth this commit surrounds refextract
  calls with try blocks to avoid the task's termination untill
  the problem is fixed.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
ftp_utils: new method

* Adds a new method to get the datestamp
  of a file.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>