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 
So we kind of already manually do this in a lot of our tasks - we return the callback with an instance of `gutil.PluginError` which handles logging err nicely as well as calling `this.emit('end')`.

Here is an example in the coffee subtask:

```js
        return function (cb) {
            // Some things omitted....
            return stream.pipe(changed(config.dest || defaults.path.buildSrc))
                .pipe(coffee({bare: config.bare || true}))
                .on('error', function(err){
                    cb(new gutil.PluginError('coffee', err));
                })
                .pipe(gulp.dest(config.dest || defaults.path.buildSrc));
```

So PluginError handles some things for us.
Perhaps we should re-evaluate this. Maybe we could get away with removing these error handlers from each task/subtask and just doing it once in this 'global' event handler...