curses: Handle an error at curses initialization.

* pinentry/pinentry-curses.c (dialog_run): Detect an error of newterm.

GPG_ERR_WINDOW_TOO_SMALL is not be the best, but it would be enough to
locate the problem.

• GnuPG-bug-id: T5623
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>