carbonserver: improve logging in fileindex
carbonserver: improve logging in fileindex

 * Make messages consistent
 * Fix error hanlding in some cases

drawks/go-carbon,drawks/go-carbon,deniszh/go-carbon,lomik/go-carbon,lomik/go-carbon,deniszh/go-carbon,drawks/go-carbon,deniszh/go-carbon,lomik/go-carbon