Echo a system bell on all errors to alert the user

Fixes #84

## Problem
Sometimes the user may not know that a wGulp watch or task has failed. 

## Solution
Alert them with a system bell.

## Testing
- Create some error scenario:
    - Missing JSX closing tag
    - Violate jshint or tslint rule
- Listen for the bell on a build

@trentgrover-wf 
@evanweible-wf 

FYI: @patkujawa-wf