Print a slightly clearer message when failing to launch a thread #57833
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As discussed in #46345, the
io::Erroryou get when a thread fails to launch is of typeio::ErrorKind::WouldBlock. This is super uninformative when an arbitrarythread::spawnfails somewhere in your code:This PR improves the situation a little bit by using
expectinstead ofunwrap. I don't consider this a complete fix for #46345 though.