Show continuation lines when doctest crashes

[user202729]

I think this makes more sense.

Normally the format is like

sage: code(code)  ## line 1234 ##
'result'

or

sage: code(code)
code(code)  ## line 1234 ##
'result'

with the patch, the display becomes

sage: code(code)
....: code(code)  ## line 1234 ##
'result'

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit bd5b622; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[user202729]

It's actually a bit difficult to test this because ....: contains ... for the ellipsis… but fine.

[user202729]

Thought about reusing one of the crash test? Or should I make a separate one?