This ensures Gulp can still catch errors without “Did you forget to signal async completion?”

Unlike Sass and npm scripts, Rollup includes error properties from Babel such as:

Details:
    code: PLUGIN_ERROR
    reasonCode: UnexpectedToken
    loc: [object Object]
    pos: 13330
    pluginCode: BABEL_PARSE_ERROR
    hook: transform

The previous commit lets Gulp hande `new PluginError()` versus logging to console, so we now need to skip typical “watch + restart” errors seen during development