some error handling

src/main.cpp

@@ -1,8 +1,23 @@
+#include <cstddef>
+#include <filesystem>
 #include <iostream>
 #include <string>
 #include "fs_reader.h"
 
 int main(int argc, char *argv[]) {
+    if (argc == 1) {
+        std::cout << "chksum: No path specified\n";
+        return 1;
+    }
+    if (argc > 2) {
+        std::cout << "chksum: Too many arguments\n";
+        return 1;
+    }
+    if (!fs::exists(argv[1])) {
+        std::cout << "chksum: Not a valid path\n";
+        return 1;
+    }
+
     std::vector<fs::path> files_and_folders = find_all_files_and_folders(argv[1]);
 
     for (size_t index = 0; index < files_and_folders.size(); index++) {