message: Log errors to stderr instead of stdout
  Replace all occurrences of console.log by console.error in logError functions.
  This allows using apollo-codegen with other commands in a script, and separating errors from normal output.
  A work-around would be to use shell redirections.
diff: @@ -28,7 +28,7 @@ export function logError(error: Error) {
  logErrorMessage(error.message, fileName);
  }
  } else {
  - console.log(error.stack);
  + console.error(error.stack);
  }
  }
  @@ -37,17 +37,17 @@ export function logErrorMessage(message: string, fileName?: string, lineNumber?:
  if (fileName && lineNumber) {
  // Prefixing error output with file name, line and 'error: ',
  // so Xcode will associate it with the right file and display the error inline
  - console.log(`${fileName}:${lineNumber}: error: ${message}`);
  + console.error(`${fileName}:${lineNumber}: error: ${message}`);
  } else {
  // Prefixing error output with 'error: ', so Xcode will display it as an error
  - console.log(`error: ${message}`);
  + console.error(`error: ${message}`);
  }
  } else {
  if (fileName) {
  const truncatedFileName = '/' + fileName.split(path.sep).slice(-4).join(path.sep);
  - console.log(`...${truncatedFileName}: ${message}`);
  + console.error(`...${truncatedFileName}: ${message}`);
  } else {
  - console.log(`error: ${message}`);
  + console.error(`error: ${message}`);
  }
  }
  }
  