Include error report about re-exporting a variable in Web version

Re-exporting vars like the following does not work:

``` javascript

require: 'A';

exports A, B

var B = function(){};

```

But this works:

``` javascript

exports A, B

var A = require('A').A;

var B = function(){};

```

The cli, I've been told, would have reported an error explaining this.
