libext2fs: reserve the error code EXT2_ET_NO_GDESC

libext2fs: reserve the error code EXT2_ET_NO_GDESC

This is really only needed in the 1.46+ where the EXT2_FLAG_SUPER_ONLY
is honored by ext2fs_open to only read the superblock, so that
fs->group_desc can be NULL.  We define it in the maint branch so that
we can be sure the error tables are kept in sync (in the unlikely case
that a new error code needs to be assigned in the maint branch).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>