If available uses ISOcodes::ISO_3166
to validate country code.
Otherwise assume that the code is valid as long as it is a two-letter code or
__
. See rr_publisher_country()
for use of country codes.
validate_country_code(country)
[character(1)
]
a two-letter country code or AA
, ZZ
or
__
(special country codes for SHERPA/RoMEO)
TRUE
if the country code is valid, errors otherwise