fix undefined start conditions error : https://github.com/zaach/jison-le...

An attempt to fix this issue https://github.com/zaach/jison-lex/issues/9

**Situation before the fix :**
If a start condition is not defined in headers the lexer fails without any error message

**Situation with this fix :**
If there is a missing start condition, the lexer triggers a console warning explaining that a start condition is not declared, and the missing start condition is declared as not inclusive (fallback behaviour)