NullPointerException for undeclared AST types used for inheritance; Proper error message missing

[Christoff Bürger]

If in an AST specification a node type is used for inheritance that is never declared, JastAdd throws a NullPointer exception instead a proper error message.

Minimal error example: abstract A; B:AA; // NullPointer exception instead error message that AA is unknown.

[Jesper Öqvist]

Added test for this in the test repository: https://bitbucket.org/jastadd/jastadd-test/commits/f7df5bd63850a6fba36da06c7306fdea0433f1c7

[Jesper Öqvist]

Fixed NPE during early isRoot test

Calling isRoot on a type that extends an unknown type no longer raises a NullPointerException.

fixes #229 (bitbucket)

→ <<cset d9e936e356e8>>