distcp overwrites files even when there are missing checksums

If distcp can't read the checksum files for the source and destination files-- for any reason-- it ignores the checksums and overwrites the destination file. It does produce a log message, but I think the correct behavior would be to throw an error and stop the distcp.

If the user really wants to ignore checksums, he or she can use -skipcrccheck to do so.

The relevant code is in DistCpUtils#checksumsAreEquals:

    try {
      sourceChecksum = sourceFS.getFileChecksum(source);
      targetChecksum = targetFS.getFileChecksum(target);
    } catch (IOException e) {
      LOG.error("Unable to retrieve checksum for " + source + " or " + target, e);
    }