better detect syntax errors in lexer.rs (#1771)

src/lib/charsio.pl

@@ -193,6 +193,7 @@ get_single_char(C) :-
 % ```
 read_from_chars(Chars, Term) :-
     must_be(chars, Chars),
+    must_be(var, Term),
     '$read_term_from_chars'(Chars, Term).
 
 %% write_term_to_chars(+Term, +Options, -Chars).

src/parser/lexer.rs

@@ -859,7 +859,13 @@ impl<'a, R: CharRead> Lexer<'a, R> {
 
                     self.get_single_quoted_char()
                         .map(|c| Token::Literal(Literal::Fixnum(Fixnum::build_with(c as i64))))
-                        .or_else(|_| {
+                        .or_else(|err| {
+                            match err {
+                                ParserError::UnexpectedChar('\'', ..) => {
+                                }
+                                err => return Err(err),
+                            }
+
                             self.return_char(c);
 
                             i64::from_str_radix(&token, 10)