No error message reported from nbfroont when there is a scanner error in the parser specification

[Jesper Öqvist]

nbfront does not report an error for the following faulty parser specification, it just exits without generating a parser:

expr =
    ID.a ADD.op IDexpr.b { return new Nonterminal("expr", a, op, b); :}

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[Jesper Öqvist]

nbfront: don't exit quietly if there is an unexpected exception

fixes #6

→ <<cset 7d38088e817d>>