node: skip errors on optional dependencies

[proletarius101]

e.g. In https://raw.githubusercontent.com/lensapp/lens/master/package-lock.json

    "node_modules/npm/node_modules/@colors/colors": {
      "version": "1.5.0",
      "inBundle": true,
      "license": "MIT",
      "optional": true,
      "engines": {
        "node": ">=0.1.90"
      }
    },

[refi64]

Should this at least be printing out a warning? Not sure if we do that anywhere else in the source yet, but it could be useful if the user was actually expecting the optional dep to still make its way inside.

[bbhtt]

@proletarius101 can you add a warning here saying something like "this {package} was skipped due to being optional"