Handle errors from execve() in the Unix process backend more gracefully.

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

changes/bug31810

@@ -0,0 +1,4 @@
+  o Minor bugfixes (process management):
+    - Remove assertion in the Unix process backend. This assertion would trigger
+      when a new process is spawned where the executable is not found leading to
+      a stack trace from the child process. Fixes bug 31810; bugfix on 0.4.0.1-alpha.