Fix for Failed Symlink on NPM Installation of 'tfjs-node'

[PhillipAWells]

Attempting to install tfjs-node or tfjs-node-gpu on the Windows platform would always result in the following error:

> node scripts/install.js

CPU-windows-3.2.0.zip
* Downloading libtensorflow

* Building TensorFlow Node.js bindings
symlink ./lib/napi-v7 failed:  null

This is due to the original code not checking the error value on the result of the symlink call. A simple conditional on whether error is null or not fixes the problem.

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This change is 

[pyu10055]

Thanks!