Fail early if duplicated labels are present

[ppm1337]

Description

Recently, I found myself rendering lengthy analysis reports using the jupyter engine. After executing all cells without errors, the render process fails complaining about duplicated labels. It becomes worse by the error message only reporting the first duplicated label it encounters. Hence, if you have multiple duplicates you may end up with the same error after fixing the initially reported duplicate.

This can quickly happen by copy and pasting existing code chunks.

In any case, I believe this error should instead be a warning and if it's really worth to be an error, the uniqueness of labels should be checked before all chunks are being executed.

Here is how this would typically look like for quarto render my_notebook.qmd:

Starting python3 kernel...Done

Executing 'my_notebook.quarto_ipynb'
  Cell 1/57: ''..................................Done
  Cell 2/57: 'label-a'...........................Done
  [...]
  Cell 17/57: 'label-q'..........................Done
  [...]
  Cell 24/57: 'label-q'..........................Done
  [...]
  Cell 57/57: 'label-xyz'........................Done

ERROR: Cell label names must be unique (found duplicate '#label-q')

Stack trace:
    at file:///path/to/my/env/bin/quarto.js:33118:15
    at jupyterToMarkdown (file:///path/to/my/env/bin/quarto.js:35033:5)
    at eventLoopTick (ext:core/01_core.js:178:7)
    at async Object.execute (file:///path/to/my/env/bin/quarto.js:77201:21)
    at async renderExecute (file:///path/to/my/env/bin/quarto.js:123649:25)
    at async renderFileInternal (file:///path/to/my/env/bin/quarto.js:123902:35)
    at async renderFiles (file:///path/to/my/env/bin/quarto.js:123698:9)
    at async render (file:///path/to/my/env/bin/quarto.js:129506:19)
    at async _Command.actionHandler (file:///path/to/my/env/bin/quarto.js:129748:24)
    at async _Command.execute (file:///path/to/my/env/bin/quarto.js:9840:7)

Using quarto 1.8.26.