Modified serializer to throw an exception if it tries

Modified serializer to throw an exception if it tries
to read from a buffer and doesn't understand what class
it's reading.  Error messages... it's the little things
that matter.

git-svn-id: http://jmonkeyengine.googlecode.com/svn/trunk@7180 75d07b2b-3a1a-0410-a2c5-0572b91ccdca