Prevent `iconsClassesGenerator.js` from failing if icons aren't found

[julien]

The iconsClassesGenerator.js script reads files but if these files are non existent it will crash and prevent the build script from generating the ckeditor bundle.

Here's how to make it crash:

• Edit icons.json and add a new "entry" to the icons object, for example

  ...
  icons: {
     "preview": "preview"
  }
  ...

• Build ckeditor with

  ./ck.sh build

• See it fail with the following message:

  fs.js:115
      throw err;
      ^
  
  Error: ENOENT: no such file or directory, open '../node_modules/@clayui/css/lib/images/icons/preview.svg'
      at Object.openSync (fs.js:439:3)
      at Object.readFileSync (fs.js:344:35)
      at Object.<anonymous> (/home/jc/Documents/liferay-ckeditor/support/iconsClassesGenerator.js:106:22)
      at Module._compile (internal/modules/cjs/loader.js:689:30)
      at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
      at Module.load (internal/modules/cjs/loader.js:599:32)
      at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
      at Function.Module._load (internal/modules/cjs/loader.js:530:3)
      at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
      at startup (internal/bootstrap/node.js:283:19)

[julien]

Fixed by #124