Shape: don't let REPACK silently fail and try to recover

QGIS does a automatic REPACK after deleting features from shapes.  This silently fails if the shape file is opened multiple times and an leaves the layer behind without a DBF.  This pull request add an error if that happens and reopens the DBF, if it cannot be removed and cleans up the _packed.{dbf,cpg}.

See also http://hub.qgis.org/issues/7540