Invalid enclosure error, when calling getFile() in filefield_feeds_set_target()

If a feed provides a file ( i.e. in temp directory) which actually does not exists there will be an error thrown by FeedsEnclosure::getFile() called in filefield_feeds_set_target(). This error won't be caught until
FeedsNodeProcessor::process(). The result is that the feed-item with an invalid filename won't be imported at all.

Using feeds to migrate for example old websites its very likely that some referenced files aren't available any more. I'd prefer to import the item anyway but without the referenced file.

See also: #926616: FeedsEnclosure::getFile() should allow files to exist in the temp directory