Fix issue with static-only unpickling raised in PR #252
Removes test, to be re-added again
Add sealed-trait-static test

This tests that we can handle a sealed trait in static-only
mode.

This test is failing now, I believe due to
https://issues.scala-lang.org/browse/SI-7046
which causes knownDirectSubclasses for Fruit
to be an empty list in Tools.scala
Improve exception when we see an unknown type tag

If we don't recognize the discriminator type when
dispatching a sealed trait, throw a PicklingException
listing the type we got and the ones we would have
understood.
Reorganizes static-only test, fails wout reorg due to SI-7046
Fail due to unknown tag only for closed types
Merge pull request #254 from scala/issue-252

Fix issue with static-only unpickling raised in PR #252