Add 'in the type bitmap' to make clear where in the RR the error occurs.
Also use 'NSEC(3)' - as this code is shared between NSEC and NSEC3, the
first error used NSECx.
Technically backwards incompatible, but checking strings in errors as
bad practice (although this lib lacks library types).
Signed-off-by: Miek Gieben <email@example.com>