Missing inherited declarations cause stack trace

[Jesper Öqvist]

A stack trace is printed by JastAdd when an inherited attribute declaration is missing for some equation. It would be preferable if a tidy error message was printed instead. Example declarations that provoke a stack trace:

    inh int A.value();
    eq Node.getChild().value(int i) = 0;

[Jesper Öqvist]

Add error message for missing inh decl

Fixed stack trace printing when inherited attribute declaration is missing for an inherited equation.

fixes #189 (bitbucket)

→ <<cset c6dbef9428f7>>