Avoid null reference exception when building Sia error messages.
Avoid null reference exception when building Sia error messages.

If the resource does not provide an error response, the Response
property will be null.

This fixes #4193.

duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,mnaiman/duplicati