FIX: Broken Public Key FIle Prevents all Mounts

This changes the behaviour of `SignatureManager::LoadPublicRsaKeys()` to be a bit more permissive. If one ore more public key files cannot be read successfully, they are just skipped. If the list of public keys is not empty but none was read successfully, it will still produce an error.

Additionally this contains a regression test for the issue described in [CVM-667](https://sft.its.cern.ch/jira/browse/CVM-667).