iconv: restore verbosity with unrecognized encoding names (bug 30694)

Commit 91927b7c ("Rewrite iconv option parsing [BZ #19519]") changed the
iconv program to call __gconv_open directly instead of the iconv_open
wrapper, but the former does not set errno.  Update the caller to
interpret the return codes like iconv_open does.

(cherry picked from commit fc72b6d7)