memory leak fix, only reload css files

[nickav]

There are two issues I noticed while using this loader:

1. The reference to el is not bound correctly when calling updateCss

For example I think it suffers from this kind of issue:

function something(i) { console.log(i); }
for (var i = 0; i < 10; i++) {
  setTimeout(function() { something(i); }, 0)
}

Will print 10 10 times. If you replace var with let this works the way you would expect. That means that this method will leak orphaned link elements on the page and not remove them correctly.

2. Sometimes el will not receive the load event, again orphaning link elements.

I added an additional listener for error and made sure that the element is actually a css link. For example, this element would be duplicated on the page:

<link rel=icon type=image/png href=/img/favicon/favicon_32.png sizes=32x32>

[coveralls]

Coverage decreased (-0.06%) to 72.093% when pulling a0cef3f on hd-dg:master into 63eb466 on shepherdwind:master.

[shepherdwind]

1.3.2 published