Add an option to ignore inexistent files by the batch import

On batch import when the "File Upload" option is activated, the files referred in the column mapped to "doc.file" get attached to the target documents.

Currently when a file added in the column mapped with "doc.file" is not found the batch import returns an error and the corresponding page is not imported at all even if other data is valid. So, when the "File Upload" option is activated we must have existing files in the mapped column or an empty values otherwise the import file of the page will fail and this is not wanted in some situations.

So, this ticket is about adding the option to ignore inexistent files during the import, this way the corresponding pages are imported.