Fix failure from unusual types in hiera files.

Raise YamlgpgError if not string, array or hash value from hiera.
This allows hiera to contain boolean values (e.g. TrueClass) & continue, rather than failing to get values.

Throws back to this section, which previously was getting bypassed:
                        # If there are any exceptions with decryption, then we go on so that
                        # other backends might find a non-encrypted value.