BLOBs have a .tostring now, but test other type which do not.

bin/varnishtest/tests/v00020.vtc

@@ -388,6 +388,10 @@ varnish v1 -errvcl {Symbol 'acl' has wrong type (reserved), expected acl:} {
 	sub vcl_recv { if (client.ip ~ acl) {} }
 }
 
+varnish v1 -errvcl {Cannot convert HTTP to STRING} {
+	sub vcl_synth { set resp.http.foo = resp; }
+}
+
 server s1 {
 	rxreq
 	txresp -hdr "bar: X"

lib/libvcc/vcc_expr.c

@@ -288,15 +288,9 @@ vcc_expr_tostring(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 		(*e)->constant = constant;
 		(*e)->nstr = 1;
 	} else {
-		if ((*e)->fmt == BLOB)
-			VSB_cat(tl->sb,
-			    "Wrong use of BLOB value.\n"
-			    "BLOBs can only be used as arguments to VMOD"
-			    " functions.\n");
-		else
-			VSB_printf(tl->sb,
-			    "Cannot convert %s to STRING.\n",
-			    vcc_utype((*e)->fmt));
+		VSB_printf(tl->sb,
+		    "Cannot convert %s to STRING.\n",
+		    vcc_utype((*e)->fmt));
 		vcc_ErrWhere2(tl, (*e)->t1, tl->t);
 	}
 }