a68: use a68_error specific tag in diagnostic message

a68: use a68_error specific tag in diagnostic message

a68_error and friends still use their own upper-letter based tag
format.  We will be switching these to use the GCC standard %-based
tags for diagnostics, hopefully soonish, but in the meanwhile do not
pass a %s tag to a68_error because bad things happen.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* a68-imports.cc (a68_open_packet): Use right tag format in
	a68_error.