Alien: NotFoundError renamed into LibrarySymbolNotFoundError

The class name "NotFoundError" was too generic in Alien package - so I renamed it
into LibrarySymbolNotFoundError. I also added the missing class comment.

I hope the small change does not create too much hazzle

----------------------------------------------------------------------------------------------------------------
Name: Alien-Core-TorstenBergmann.101
Author: TorstenBergmann
Time: 14 June 2019, 9:48:43.798137 pm
UUID: 796ac3ab-af47-0d00-8f6e-47ed0d723e43
Ancestors: Alien-Core-tbn.100
-------------------------------------------------------------------------------

- done in original SqueakSource version  http://www.squeaksource.com/Alien.html as Alien-Core-TorstenBergmann.101.mcz
- and for Pharo: https://github.com/pharo-project/pharo/issues/3536 / https://github.com/pharo-project/pharo/pull/3537