change error message; require unicode codepoint

before this, it recommends the range that can be represented by utf-8
but the range of valid unicode codepoint is narrower than that. for
error message, it is good to recommend valid unicode codepoint.

toml/parser.hpp

@@ -272,7 +272,7 @@ std::string read_utf8_codepoint(const region<Container>& reg)
     {
         throw std::range_error(format_underline(concat_to_string("[error] "
                 "input codepoint (", str, ") is too large to encode as utf-8."),
-                reg, "should be in [0x00..0x1FFFFF]"));
+                reg, "should be in [0x00..0x10FFFF]"));
     }
     return character;
 }