This extracts an FFI Error type `ddog_Error`. It contains an FFI Vec
internally:

```rust
/// Please treat this as opaque; do not reach into it, and especially
/// don't write into it!
pub struct Error {
    /// This is a String stuffed into the vec.
    message: Vec<u8>,
}
```

FFI result types have been updated e.g.:

```rust
pub enum SendResult {
    HttpResponse(HttpStatus),
    Err(Error),
}
```

Instead of reaching into the buffer, use these APIs:

```c
/**
 * # Safety
 * Only pass null or a valid reference to an Error.
 */
void ddog_Error_drop(struct ddog_Error *error);

/**
 * Returns a CharSlice of the error's message that is valid until the error
 * is dropped.
 * # Safety
 * Only pass null or a valid reference to an Error.
 */
ddog_CharSlice ddog_Error_message(const struct ddog_Error *error);
```

I think API usage is more likely to be correct when each case is
dropped as needed. This shows the diff for our examples.