Fixed error methods on iOS (#551)

ios/Classes/FlutterWebviewPlugin.m

@@ -358,19 +358,27 @@ static NSString *const CHANNEL_NAME = @"flutter_webview_plugin";
     [channel invokeMethod:@"onState" arguments:@{@"type": @"startLoad", @"url": webView.URL.absoluteString}];
 }
 
+- (void)webView:(WKWebView *)webView didFailProvisionalNavigation:(WKNavigation *)navigation withError:(NSError *)error {
+    NSString* url = webView.URL == nil ? @"?" : webView.URL.absoluteString;
+    
+    [channel invokeMethod:@"onHttpError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", error.code], @"url": url}];
+}
+
 - (void)webView:(WKWebView *)webView didFinishNavigation:(WKNavigation *)navigation {
     [channel invokeMethod:@"onState" arguments:@{@"type": @"finishLoad", @"url": webView.URL.absoluteString}];
 }
 
 - (void)webView:(WKWebView *)webView didFailNavigation:(WKNavigation *)navigation withError:(NSError *)error {
-    [channel invokeMethod:@"onError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", error.code], @"error": error.localizedDescription}];
+    [channel invokeMethod:@"onHttpError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", error.code], @"error": error.localizedDescription}];
 }
 
 - (void)webView:(WKWebView *)webView decidePolicyForNavigationResponse:(WKNavigationResponse *)navigationResponse decisionHandler:(void (^)(WKNavigationResponsePolicy))decisionHandler {
     if ([navigationResponse.response isKindOfClass:[NSHTTPURLResponse class]]) {
         NSHTTPURLResponse * response = (NSHTTPURLResponse *)navigationResponse.response;
 
-        [channel invokeMethod:@"onHttpError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", response.statusCode], @"url": webView.URL.absoluteString}];
+        if (response.statusCode >= 400) {
+            [channel invokeMethod:@"onHttpError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", response.statusCode], @"url": webView.URL.absoluteString}];
+        }
     }
     decisionHandler(WKNavigationResponsePolicyAllow);
 }