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

+  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.

src/lib/process/process_unix.c

@@ -253,22 +253,15 @@ process_unix_exec(process_t *process)
     process_environment_t *env = process_get_environment(process);
 
     /* Call the requested program. */
-    retval = execve(argv[0], argv, env->unixoid_environment_block);
+    execve(argv[0], argv, env->unixoid_environment_block);
 
     /* If we made it here it is because execve failed :-( */
-    if (-1 == retval)
-      fprintf(stderr, "Call to execve() failed: %s", strerror(errno));
-
     tor_free(argv);
     process_environment_free(env);
 
-    tor_assert_unreached();
-
  error:
-    /* LCOV_EXCL_START */
     fprintf(stderr, "Error from child process: %s", strerror(errno));
     _exit(1);
-    /* LCOV_EXCL_STOP */
   }
 
   /* We are in the parent process. */