make private struct warning an error

examples/vweb/vweb_example.v

@@ -6,7 +6,7 @@ const (
 	port = 8082
 )
 
-struct App {
+pub struct App {
 pub mut:
 	vweb vweb.Context // TODO embed
 	cnt int

tools/gen_vc.v

@@ -90,7 +90,7 @@ mut:
 }
 
 // webhook server
-struct WebhookServer {
+pub struct WebhookServer {
 pub mut:
 	vweb   vweb.Context
 	gen_vc &GenVC

vlib/builtin/array.v

@@ -5,7 +5,7 @@ module builtin
 
 import strings
 
-struct array {
+pub struct array {
 pub:
 // Using a void pointer allows to implement arrays without generics and without generating
 // extra code for every type.

vlib/compiler/struct.v

@@ -342,7 +342,7 @@ fn (p mut Parser) struct_init(typ_ string) string {
 	mut typ := typ_
 	mut t := p.table.find_type(typ)
 	if !t.is_public && t.mod != p.mod {
-		p.warn('type `$t.name` is private')
+		p.error('struct `$t.name` is private')
 	}
 	// generic struct init
 	if p.peek() == .lt {