Improve error message where defaulting fails in expression

Given this code:
```
fn nop(a: &? [_], f) {
f(a)
}

let mut a = [5, 4, 11, 3, 2, 1, 0, 7];
nop(a[0], |x| x)
```

Compilation fails because the call to `nop(a[0], |x| x)` requires `a[0]` to be a `Num`, but the signature of `nop` requires a[0] to be an array. This leads to this constraint: `[A]: Num`, which cannot be satisfied. So this should be the error.

However, no error is emitted at that point and constraint solving is deferred, leading to the following error at the end: `Unbound type variable A in type [A]`.

If used in a function:
```
fn f() {
let mut a = [5, 4, 11, 3, 2, 1, 0, 7];
nop(a[0], |x| x)
}
```

No issue arises because then `f` simply has the signature: `f() -> [A] where [A]: Num`, which is a fine signature.

Ideally, we could add some smarter error messages when type defaulting fails in expressions.