When the data-formatters happen to break (e.g., due to layout changes in
libc++), there's no clear indicator of them failing from a user's
perspective. E.g., for `std::vector`s we would just show:
```
(std::vector<int>) v = size=0 {}
```
which is highly misleading, especially if `v.size()` returns a non-zero
size.
This patch surfaces the various errors that could occur when calculating
the number of children of a vector.
rdar://146964266
419fa1b06a