Avoid KeyNotFoundException

Resolves #769.

src/AspNet.Security.OAuth.Vkontakte/VkontakteAuthenticationHandler.cs

@@ -49,7 +49,19 @@ protected override async Task<AuthenticationTicket> CreateTicketAsync(
         }
 
         using var container = JsonDocument.Parse(await response.Content.ReadAsStringAsync(Context.RequestAborted));
-        using var enumerator = container.RootElement.GetProperty("response").EnumerateArray();
+
+        if (!container.RootElement.TryGetProperty("response", out var profileResponse))
+        {
+            if (container.RootElement.TryGetProperty("error", out var error) &&
+                error.ValueKind is JsonValueKind.String)
+            {
+                throw new InvalidOperationException($"An error occurred while retrieving the user profile: {error.GetString()}");
+            }
+
+            throw new InvalidOperationException("An error occurred while retrieving the user profile.");
+        }
+
+        using var enumerator = profileResponse.EnumerateArray();
         var payload = enumerator.First();
 
         var principal = new ClaimsPrincipal(identity);