releasetools: Change the base_fs assertion into warnings.

    if "system_base_fs_file" in d:

      basename = os.path.basename(d["system_base_fs_file"])

      system_base_fs_file = os.path.join(input_dir, "META", basename)

      assert os.path.exists(system_base_fs_file), \

          "failed to find system base fs file: %s" % (system_base_fs_file,)

      if os.path.exists(system_base_fs_file):

        d["system_base_fs_file"] = system_base_fs_file

      else:

        print "Warning: failed to find system base fs file: %s" % (

            system_base_fs_file,)

        del d["system_base_fs_file"]

    if "vendor_base_fs_file" in d:

      basename = os.path.basename(d["vendor_base_fs_file"])

      vendor_base_fs_file = os.path.join(input_dir, "META", basename)

      assert os.path.exists(vendor_base_fs_file), \

          "failed to find vendor base fs file: %s" % (vendor_base_fs_file,)

      if os.path.exists(vendor_base_fs_file):

        d["vendor_base_fs_file"] = vendor_base_fs_file

      else:

        print "Warning: failed to find vendor base fs file: %s" % (

            vendor_base_fs_file,)

        del d["vendor_base_fs_file"]

  try:

    data = read_helper("META/imagesizes.txt")