Check whether files could be opened before writing; resolves #542 (#555)

Author: liuzicheng1987

include/rfl/io/save_bytes.hpp

@@ -15,6 +15,9 @@ Result<Nothing> save_bytes(const std::string& _fname, const T& _obj,
                            const WriteFunction& _write) {
   try {
     std::ofstream output(_fname, std::ios::out | std::ios::binary);
+    if (!output.is_open()) {
+      return error("Unable to open file '" + _fname + "' for writing.");
+    }
     _write(_obj, output);
     output.close();
   } catch (std::exception& e) {

include/rfl/io/save_string.hpp

@@ -15,6 +15,9 @@ Result<Nothing> save_string(const std::string& _fname, const T& _obj,
   try {
     std::ofstream outfile;
     outfile.open(_fname);
+    if (!outfile.is_open()) {
+      return error("Unable to open file '" + _fname + "' for writing.");
+    }
     _write(_obj, outfile);
     outfile.close();
   } catch (std::exception& e) {