Reject n_jobs=0 in MultiprocessingBackend before the nesting checks

[ckarnell]

SequentialBackend, PoolManagerMixin and LokyBackend all reject n_jobs=0 as their first act. MultiprocessingBackend gets there last, after the daemon and nesting checks, so in a daemonic process the invalid value is swallowed and reported as a nesting problem instead.

>>> # inside a daemonic process, e.g. a worker
>>> with parallel_config(backend='multiprocessing'):
...     joblib.effective_n_jobs(0)
UserWarning: Multiprocessing-backed parallel loops cannot be nested, setting n_jobs=1
1

The other three raise there. ValueError: n_jobs == 0 in Parallel has no meaning. effective_n_jobs is public and in __all__, so this is the documented error going missing on one backend.

test_invalid_njobs already asserts the behaviour for every backend and passes today, because it goes through Parallel. When the backend returns 1, configure raises FallbackToBackend(SequentialBackend(...)), and the sequential backend then raises for its own reasons. The error a caller sees is right, and it's arriving from a different backend than the one they asked for.

Three lines. Nothing moves. The check is added at the top instead of reordering what's there, so the daemon and nesting branches behave exactly as before for every valid n_jobs.

Test is test_invalid_njobs_in_daemon_process, parametrized over the four backends and going through the public effective_n_jobs instead of Parallel, since Parallel recovers and hides it. Reverting _parallel_backends.py fails it on multiprocessing alone with DID NOT RAISE, and the other three pass either way. test_parallel.py is 575 passed, 24 skipped, 3 xpassed, against 574 and the one failure on revert.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.07%. Comparing base (804f472) to head (47c17de).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1838   +/-   ##
=======================================
  Coverage   94.06%   94.07%           
=======================================
  Files          46       46           
  Lines        8070     8082   +12     
=======================================
+ Hits         7591     7603   +12     
  Misses        479      479           

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[Nanored4498]

LGTM. As some users might currently use the effective_n_jobs(0) with 'multiprocessing', I prefer waiting for another reviewer opinion before merging.