allow `build` and `watch` tasks to run if required folders not found

[jonknapp]

Currently, if you don't have the folders expected by the build and watch tasks, they will fail. If a project does not require static files or html compilation, we should fail to build their project.

The folders expected by slipcast and their default values can be found in config/slipcast.js:

  folders: {
    css: 'app/css',
    javascript: 'app/js',
    pages: 'app/pages',
    static: 'app/static',
    views: 'app/views',
  }

1. We should allow scripts/build/css.js file (with watch and build commands) to run without a config.folders.css folder as long as there are no css/sass files mentioned in the slipcast config.
2. We should allow scripts/build/js.js file (with watch and build commands) to run without a config.folders.js folder as long as there are no js files mentioned in the slipcast config.
3. We should allow scripts/build/html.js file (with watch and build commands) to run if both config.folders.pages and config.folders.views are not available.
4. We should allow the build task to complete if config.folders.static is not available.