Error handling for VATSIM Connect

[sentry-io]

Sentry Issue: VATPRC-UNIAPI-F

Flurl.Http.FlurlHttpException: Call failed with status code 400 (Bad Request): POST https://auth.vatsim.net/oauth/token
  File "FlurlClient.cs", line 337, in async Task<IFlurlResponse> FlurlClient.HandleExceptionAsync(FlurlCall call, Exception ex, CancellationToken token)
    throw ex;
  File "FlurlClient.cs", line 183, in async Task<IFlurlResponse> FlurlClient.SendAsync(IFlurlRequest request, HttpCompletionOption completionOption, CancellationToken cancellationToken) x 2
    return await HandleExceptionAsync(call, ex, cancellationToken).ConfigureAwait(false);
  File "ResponseExtensions.cs", line 21, in async Task<T> ResponseExtensions.ReceiveJson<T>(Task<IFlurlResponse> response)
    using var resp = await response.ConfigureAwait(false);
  File "Services/VatsimAuthService.cs", line 76, in async Task<TokenResponse> VatsimAuthService.GetTokenAsync(string code, string verifier)
  File "Controllers/AuthController.cs", line 362, in async Task<IActionResult> AuthController.VatsimCallback(string code, string state, string error)