Replaced DOMException.QUOTA_EXCEEDED_ERR with its value

Replaced the constant DOMException.QUOTA_EXCEEDED_ERR with its value in order to
support IE8, since DOMException is not defined in IE8 and below.

An alternative would be to check if DOMException is defined and set it if not.

Would close issue https://github.com/jeromegn/Backbone.localStorage/issues/93