Improve error message in tgt-vvp/eval_expr.c:draw_eval_expr_wid().

The IVL_EX_NONE type was not displaying any message and default did not
use the new ivl_expr_{file,lineno} functions to get the source location.
This patch remedies these problems.

tgt-vvp/eval_expr.c

@@ -2332,9 +2332,10 @@ struct vector_info draw_eval_expr_wid(ivl_expr_t exp, unsigned wid,
 
       switch (ivl_expr_type(exp)) {
 	  default:
-	    fprintf(stderr, "vvp error: unhandled expr type: %u\n",
-	    ivl_expr_type(exp));
 	  case IVL_EX_NONE:
+	    fprintf(stderr, "%s:%u:  vvp-tgt error: unhandled expr. type: "
+	            "%u at %s:%d\n", ivl_expr_file(exp), ivl_expr_lineno(exp),
+                    ivl_expr_type(exp), __FILE__, __LINE__);
 	    assert(0);
 	    res.base = 0;
 	    res.wid = 0;