gh-141860: Add on_error= keyword arg to `multiprocessing.set_forkserver_preload`...

gh-141860: Add on_error= keyword arg to `multiprocessing.set_forkserver_preload` (GH-141859)

Add a keyword-only `on_error` parameter to `multiprocessing.set_forkserver_preload()`. This allows the user to have exceptions during optional `forkserver` start method module preloading cause the forkserver subprocess to warn (generally to stderr) or exit with an error (preventing use of the forkserver) instead of being silently ignored.

This _also_ fixes an oversight, errors when preloading a `__main__` module are now treated the similarly. Those would always raise unlike other modules in preload, but that had gone unnoticed as up until bug fix PR GH-135295 in 3.14.1 and 3.13.8, the `__main__` module was never actually preloaded.

Based on original work by Nick Neumann @aggieNick02 in GH-99515.