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

[GCheung55]

Re-exporting vars like the following does not work:

require: 'A';

exports A, B

var B = function(){};

But this works:

exports A, B

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

var B = function(){};

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

[sebmarkbage]

Note to self: Also add a warning if "require:" is used to import a module that has non-lexical exports.