* tools/gpgtar-create.c (gpgtar_create): Return an error code, fix
error handling.
* tools/gpgtar-extract.c (gpgtar_extract): Likewise.
* tools/gpgtar-list.c (read_header): Return an error code.
(gpgtar_list): Return an error code, fix error handling.
(gpgtar_read_header): Return an error code.
* tools/gpgtar.c: Add missing include.
(main): Print an generic error message if a command failed and no
error has been printed yet.
* tools/gpgtar.h (gpgtar_{create,extract,list,read_header}): Fix the
prototypes accordingly.