ApiException: add synchronous [Try]GetContentAs

[jods4]

Problem statement
When catching an ApiException, you may want to look at the response body as it could be a well known Error format.
Of course this peeking might fail: no body or not the right format.

ApiException offers T? GetContentAsAsync<T>() to deserialize the error contents.
At first glance it looks good, but in practice it falls short.

The problem is that this method can throw, or return null.
When it happens you want to fallback to a more generic catch handler, which isn't easily doable (no goto!).
The obvious solution would be to put GetContentAsAsync() in an exception filter (aka catch .. when ..).
That does not work because you can't await inside an exception filter, CLR requires synchronous filters.

Using only plain Refit API, you end up with code like this...

try
{
  try
  {
    // call a Refit API
    var response = await client.DoStuff();
  }
  catch (ApiException ex)
  {
    var error = await ex.GetContentAsAsync<Error>();
    if (error is null) throw;
    // Do Error-specific stuff
    return;
  }
}
catch (Exception ex)
{
  // Do generic exception stuff
}

Suggested solution
As the response is already buffered in a string, GetContentAs may well be sync.
(This implies one new sync method on IHttpContentSerializer).
It would also be nice if the result is non-nullable and it wouldn't throw on errors, maybe bool TryGetContentAs<T>(out T result).
(Those two requirements are not hard-requirements: 1. an exception filter that throws is equivalent to false; 2. one could do when (ex.GetContentAs<T>() is T error) to filter out nulls.)

With this, plain Refit code becomes a lot more intuitive and readable:

try
{
}
catch (ApiException ex) when (ex.TryGetContentAs<Error>(out var error))
{
  // Do something with `error`
}
catch (Exception ex)
{
  // Non-specific error handling
}