feat: The error message should be printed whenever there is an error in `render_single_qmd()`

Description

The error message is not printed here :

  # try rendering qmd and warn user if successful / fail
  tryCatch(
    expr = {
      quarto_render(
        input = qmd,
        metadata = c(metadata, list(`extract-media` = img_dir)),
        output_format = output_format,
        quiet = quiet
      )
      return(TRUE)
    },
    error = \(error_message) {
      cli_alert_danger("Failed to render {qmd}, cleaning and existing")

      # throw an error with original error message
      return(FALSE)
    }
  )

It should be printed, maybe if debug is TRUE?

Start at the render_single_qmd() error handler shown in the issue and inspect how the caught error is currently discarded. Determine whether the original error message should always be surfaced or only when debug is enabled. Done means failed renders expose the underlying error while retaining the existing failure behavior; no test file is named in the issue.