Avoid allocating in pre_exec closure

[purplesyringa]

Error::other allocates memory (see rust-lang/rust#148971) and thus isn't async-signal-safe. last_os_error is. It isn't possible to transmit the error message across pre_exec, unless you're willing to write to stderr, but the old method swallowed the text too, so hopefully that isn't a problem.

[MaxVerevkin]

Thanks!