title: ServerCertificateCustomValidationCallback customize exception
body: <p>I want to do custom certificate validation via ServerCertificateCustomValidationCallback property of HttpClientHandler.  However, the callback provided can only return true (for valid certificate) or false (for invalid certifcate).  If the certificate is rejected, on the client i get a standard error message:</p>
  <p>&quot;The remote certificate was rejected by the provided RemoteCertificateValidationCallback&quot;</p>
  <p>which is not very informative.</p>
  <p>I would like to give more information in the error message as to why the certificate is invalid, e.g. hostname mismatch, missing certificate, untrusted root, revoked, etc., in order to make it easier for the user to diagnose the problem.  Is it possible to relay this information from the callback somehow?  In particular, can I throw an exception from the callback instead of returning false?  If not, what could be another way to do this?</p>