[Build Tools] Fixes docs build error from Node's fs

[whoisryosuke]

Building docs is broken. This fixes it.

• Cloned a fresh copy of SUI
• Ran npm i
• Ran gulp build-docs and received error.

fs from Node now requires a callback for the third parameter, which was missing from the docs Gulp build task file, causing the gulp build-docs process to break with the following error:

[15:59:18] Starting 'build-docs'...
Building Metadata
Copying examples
Copying LESS source
Building Semantic for docs
(node:72733) [DEP0013] DeprecationWarning: Calling an asynchronous function without callback is deprecated.
fs.js:113
      throw err;  // Forgot a callback but don't know where? Use NODE_DEBUG=fs
      ^

Error: ENOENT: no such file or directory, open '/Users/username/Development/HTML/docs/out/metadata.json'
ITs-MacBook-Pro-6:ksc-ui photoroom$ ./node_modules/gulp/bin/gulp.js build-docs

Adding a simple error callback (with optional console log for error reporting) fixes this issue. Running the docs build process now works.

Not sure if this has already been hashed out for the next release cycle.