Internal helper function for checking specific arguments used in custom_search.
check_args_date(date_char, negate)A character vector with format "DD-Mon-YYYY", e.g. "01-Apr-2019". We opted not to use objects of type "date", since IMAP servers like this not so common date format.
If TRUE, negates the search and seeks for
"NOT search_criterion". Default is FALSE.
NULL if arguments are correct.
Other customsearch checkargs functions: check_args_flag,
check_args_size,
check_args_string,
check_args_within