private String uriCacheKey(final Uri baseUri, final List<String> subdirs, final int max) {
        return UriUtils.getPseudoUriString(baseUri, subdirs, max);
    }
Gets the Folder File; creates it if necessary and performs all possible error handlings.
This is used for locations of Type DOCUMENT and FILE
<br
This method is in many respects the core method of this class

@param folder folder to get file for
@return file folder, or null if creation/retrieving was not at all possible
main/src/main/java/cgeo/geocaching/storage/DocumentContentAccessor.java