example: Hint at possible problem when binaries are missing

Helpful when forgetting to build necessary targets.

Author: hodlinator

example/example.cpp

@@ -30,6 +30,8 @@ static auto Spawn(mp::EventLoop& loop, const std::string& process_argv0, const s
         fs::path path = process_argv0;
         path.remove_filename();
         path.append(new_exe_name);
+        if (!fs::exists(path))
+            throw std::runtime_error{std::string{path} + " is missing, probably needs to be built?"};
         return {path.string(), std::to_string(fd)};
     });
     return std::make_tuple(mp::ConnectStream<InitInterface>(loop, fd), pid);