Improve error handling and update OCR API JSON key from "latex_styled" to "text"

[joliss]

This code in convert.js

    try {
      fs.writeFileSync(destination, responseJSON["text_display"]);
    } catch (err) {
      console.log(`Could not write to destination path: ${destination}`);
      return process.exit(1);
    }

just prints "Could not write to destination path" when the "text_display" key is missing. This PR improves the error handling and updates the JSON key.

This is a bug report more so than a PR -- I won't have the bandwidth to update this PR, but feel free to use it as a starting point.