gpg-interface: mark a string for translation

gpg-interface: mark a string for translation

Previous commits have marked a number of error or warning messages in
"builtin/fast-export.c" and "builtin/fast-import.c" for translation.

As "gpg-interface.c" code is used by the fast-export and fast-import
code, we should make sure that error or warning messages are also all
marked for translation in "gpg-interface.c".

To ensure that, let's mark for translation an error message in a
die() function.

With this, all the error and warning messages emitted by fast-export
and fast-import can be properly translated.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>