Error message for already specialized type is misleading

[apinski-cavium]

Take:

header t<tt>
{
  bit y;
}

typedef t ty;
typedef ty<bit> tty;
typedef tty<bit> tty1;

Currently we get:

t2.p4(8): [--Werror=type-error] error: tty<bit<1>>: Type header t<...> is not generic and thus it cannot be specialized using type arguments
typedef tty<bit> tty1;
        ^^^^^^^^
t2.p4(7)
typedef ty<bit> tty;
        ^^^^^^^

But this is misleading and should be improved saying tty is already specialized so it cannot be specialized using type arguments or something to that effect.