Expose exception to user as :throwable

This is related to #51.
The root cause of the issue was that clojure datastructures implement `Serializable` interface without having a serialVersionUID. I had filed a JIRA for this earlier http://dev.clojure.org/jira/browse/CLJ-1327.
With this change the error becomes evident to user of nippy which is otherwise ignored.

```clj
{:nippy/unthawable "org.Dummy"
 :type :serializable
 :cause "clojure.lang.Keyword; local class incompatible: stream classdesc serialVersionUID = 412129
1061259229960, local class serialVersionUID = -6496198289950458564"}
```