Better error message in stack alloc for stack ptr

[sarranz]

This program

export fn main() -> () {
    stack u64 x;
    x = 0;
    [x] = 0;
}

gives

compilation error in function main:
stack allocation: x (defined at "ble.jazz", line 3 (4-5)) is a stack variable,
but a reg variable is expected

which makes sense.

But this one

export fn main() -> reg u64 {
    stack u64[1] a;
    stack ptr u64[1] pa;
    pa = a;
    pa[0] = 0;
    reg u64 r;
    r = pa[0];
    return r;
}

gives

compilation error in function main:
stack allocation: pa (defined at "ble.jazz", line 4 (4-6)) is a stack pointer, it should
not appear in an expression

where it's not clear that the problem is that it is stack instead of reg, and also not clear where the expression part comes from (I can't tell from the code either). Doesn't the first error message work for both cases?