ignore missing ib_logfile* when using --data-source=<directory>

- Previously -D <directory> would check for an ib_logfile0 as a
  sanity check whether that path was in use. If ib_logfile0 was
  not present, dbsake would stack trace.  Now dbsake will silently
  ignore the error and continue anyway - this might be the case
  for a brand new datadir created via mysql_install_db or if
  someone is running with skip-innodb.
- Resolves issue #49