Avoid NPE on certificates that cannot be read

Skip certificates in a DirectoryCertificateSource that cannot be read to
due IOExceptions or CertificateExceptions, this prevents a NPE but
connections will still fail due to the certificate being unusable and no
valid trust-anchor existing.

This also logs the error since this really shouldn't happen.
Bug: 29997695

Change-Id: I9f7327efc302a259fb951f1f61f7fc4d647821fa