anchor.diff:  
     var readme = args['sdk-readme'];
     if (readme != null && !(new File(readme).existsSync())) {
  -    print("Warning: unable to locate the SDK description file at $readme.");
  +    print("Error: unable to locate the SDK description file at $readme.");
       exit(1);
     }
   
     Directory inputDir = new Directory(args['input']);
     if (!inputDir.existsSync()) {
  -    print("Warning: unable to locate the input directory at ${inputDir.path}.");
  +    print("Error: unable to locate the input directory at ${inputDir.path}.");
       exit(1);
     }
   
  
positive.code:   String url = args['hosted-url'];
    String footerFilePath = _resolveTildePath(args['footer']);
    if (footerFilePath != null && !new File(footerFilePath).existsSync()) {
      print(
          "Warning: unable to locate the file with footer at ${footerFilePath}.");
      exit(1);
    }
    String headerFilePath = _resolveTildePath(args['header']);
    if (headerFilePath != null && !new File(headerFilePath).existsSync()) {
      print(
          "Warning: unable to locate the file with header at ${headerFilePath}.");
      exit(1);
    }
  
  
negative.code:   Directory packageRootDir;
    if (args['package-root'] != null) {
      packageRootDir = new Directory(_resolveTildePath(args['package-root']));
    }
  
    if (args.rest.isNotEmpty) {
      var unknownArgs = args.rest.join(' ');
      print('Error: detected unknown command-line argument(s): $unknownArgs');
      _printUsageAndExit(parser, exitCode: 1);
    }
  
    PackageMeta packageMeta = sdkDocs
  