message: elle.from-string: improve error message
  Suggested by Matthieu Nottale.
diff: @@ -19,14 +19,14 @@ namespace elle
  {
  template <typename Type>
  auto
  - check_non_negative(std::string const& s)
  + check_unsigned(std::string const& s)
  {
  meta::static_if<std::is_unsigned<Type>{}>
  ([&s]
  {
  for (auto c: s)
  if (c == '-')
  - err<std::out_of_range>("invalid positive number: %s", s);
  + err<std::out_of_range>("invalid unsigned number: %s", s);
  else if (!std::isspace(c))
  break;
  })();
  @@ -38,7 +38,7 @@ namespace elle
  Type \
  from_string(std::string const& s, Type*) \
  { \
  - check_non_negative<Type>(s); \
  + check_unsigned<Type>(s); \
  auto size = size_t{}; \
  auto res = Conversion(s, &size); \
  if (size < s.size()) \
  