Add logs for FileSystem vaults when cert is not found for alias

[bmaidics]

Currently if Zilla is misconfigured to use an alias that doesn't exists in the the FileSystem vault truststore, we get a weird SSL debug exception only: java.lang.RuntimeException: Unexpected error: java.security.InvalidAlgorithmParameterException: the trustAnchors parameter must be non-empty
Zilla should log if the certification for the configured alias is not found in the truststore.
(Only log in debug mode)