plt: Throw an appropriate error for missing $var

plt: Throw an appropriate error for missing $var

* tapsets.cxx (plt_expanding_visitor::visit_target_symbol): Throw a
  semantic_error for missing $var, so it may be caught and chained for
  later.  This ensures that the visitor always has a provide() too.
* testsuite/semko/plt1.stp: New, check that missing $var is an error.
* testsuite/semok/plt1.stp: New, check that $$name works, as does
  @defined on a missing $var.