cmd/ebitenmobile: Define an error handler for iOS

Updates #919

cmd/ebitenmobile/gobind.go

@@ -122,6 +122,7 @@ const objcM = `// Code generated by ebitenmobile. DO NOT EDIT.
 
 @implementation {{.PrefixUpper}}EbitenViewController {
   GLKView* glkView_;
+  bool error_;
 }
 
 - (GLKView*)glkView {
@@ -170,13 +171,23 @@ const objcM = `// Code generated by ebitenmobile. DO NOT EDIT.
 }
 
 - (void)glkView:(GLKView*)view drawInRect:(CGRect)rect {
+  if (error_) {
+    return;
+  }
   NSError* err = nil;
   EbitenmobileviewUpdate(&err);
   if (err != nil) {
-    NSLog(@"Error: %@", err);
+    [self performSelectorOnMainThread:@selector(onErrorOnGameUpdate:)
+                           withObject:err
+                        waitUntilDone:NO];
+    error_ = true;
   }
 }
 
+- (void)onErrorOnGameUpdate:(NSError*)err {
+  NSLog(@"Error: %@", err);
+}
+
 - (void)updateTouches:(NSSet*)touches {
   for (UITouch* touch in touches) {
     if (touch.view != [self glkView]) {

cmd/ebitenmobile/main.go

@@ -212,6 +212,7 @@ const objcH = `// Code generated by ebitenmobile. DO NOT EDIT.
 #import <UIKit/UIKit.h>
 
 @interface {{.PrefixUpper}}EbitenViewController : UIViewController
+- (void)onErrorOnGameUpdate:(NSError*)err;
 @end
 `