[SPARK-4080] Only throw IOException from [write|read][Object|External]

If classes implementing Serializable or Externalizable interfaces throw
exceptions other than IOException or ClassNotFoundException from their
(de)serialization methods, then this results in an unhelpful
"IOException: unexpected exception type" rather than the actual exception that
produced the (de)serialization error.

This patch fixes this by adding a utility method that re-wraps any uncaught
exceptions in IOException (unless they are already instances of IOException).