Catch exception properly

`\OCA\Files\Helper::buildFileStorageStatistics` might throw an exception from `OC_Helper::getStorageInfo`, previously this lead to a uncatched exception being thrown when invoking this methods.

This was user triggable by for example calling `/index.php/apps/files/ajax/delete.php` with a not existing dir (for example `dir=asdf/../&allfiles=true`)