Better ExtractFromString

- Throw a exception if we cannot extract from string.
  (We throw the same exception as `std::sto*`)
- Add a specialization to extract string from string
- Add some unit test

src/tools/stringTools.cpp

@@ -405,3 +405,8 @@ std::vector<std::string> kiwix::getTitleVariants(const std::string& title) {
   variants.push_back(kiwix::toTitle(title));
   return variants;
 }
+
+template<>
+std::string kiwix::extractFromString(const std::string& str) {
+  return str;
+}