This patch removes a call to tor_assert_unreached() after execve()
failed. This assertion leads to the child process emitting a stack trace
on its standard output, which makes the error harder for the user to
demystify, since they think it is an internal error in Tor instead of
"just" being a "no such file or directory" error.
The process will now instead output "Error from child process: X" where
X is the stringified version of the errno value.
See: https://bugs.torproject.org/31810
