🐛 Warn on needservice naming an unregistered service instead of crashing

[havinash-007]

Closes #2101

What

A needservice naming an unregistered service ended the build with a raw traceback: ServiceManager.get raises NeedsServiceException, which derived from BaseException and was not caught in NeedserviceDirective.run, so it also bypassed Sphinx's own except Exception handling.

• NeedserviceDirective.run now catches it and reports a located needs.directive warning (same text: the service name and the registered ones), then returns no nodes so the build continues (-W fails it like any other warning).
• NeedsServiceException now derives from Exception (as suggested in the issue; nothing in the repo relies on it bypassing except Exception).
• Regression test tests/test_service_unknown.py (fails before with the exception, passes after) and a changelog entry.

Checks

• Tests added (test_service_unknown.py; fails before the change, passes after)
• Changelog entry in docs/changelog.rst
• uv run poe lint (all prek hooks, incl. ruff and ty) passes
• uv run pytest packages/sphinx-needs/tests -k service passes
• Full sphinx-needs suite: same result as before the change apart from the new passing test. 121 failures and 46 errors occur identically on a clean checkout (all PlantUML rendering, no renderer in my environment)
• Docs: no user-facing documentation page needed changing

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[chrisjsewell]

Thanks for the quick fix. #2101 is now fixed through #2113, so I'm closing this one in its favour.

#2113 fixes it together with #2067: the registration check there means a service whose class cannot be registered is skipped with a warning, and a needservice naming it, or naming a service that never existed, takes the same path, a located needs.load_service_need warning with the existing not-found text, with NeedsServiceException an Exception as here. Appreciated.