improve error message when `tbl-subcap` is used incorrectly

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

---
title: "tbl-subcap"
format: html
---

```{r}
#| label: df
df <- tibble::tibble(ltrs = letters[1:3], ints = 1:3)
```

```{r}
#| label: tbl-df
#| tbl-cap: "Data Frame df"
#| tbl-subcap: "Various values"

df
```

The following error was generated:

  - ERROR (C:\PROGRA~1\RStudio\RESOUR~1\app\bin\quarto\share\pandoc\datadir\_json.lua:212) unexpected character 'V' at line 1 col 1


### Steps to reproduce

_No response_

### Actual behavior

_No response_

### Expected behavior

_No response_

### Your environment

_No response_

### Quarto check output

```{r}
quarto::quarto_version()
```
[1] ‘1.7.32’

Reproduce the provided R/Quarto document using tbl-subcap and inspect the reported _json.lua:212 failure during rendering. Trace how the invalid subcaption reaches that parser and make the resulting error identify the misuse clearly, then verify the same example no longer produces the opaque parse error.