Put KeyPairPersistence load into a try{}catch{} block.

Kpp.Load() will throw an unhandled exception if _filename (a folder to use as the key store) does not exist and it cannot be created.

This is an entirely valid thing to have happen - cert importing into the machine store (mozroots --import --machine --sync) as root will create a /usr/share/.mono/certs/ folder, but will not create /usr/share/.mono/keypairs - and since the root user owns /usr/share/.mono/, if the user tries to enumerate the certificates in /usr/share/.mono/certs/, they get 0 certificates returned due to the unhandled exception dropping each loaded certificate on the floor.

Closes: Xamarin bug 23015