Handle unterminated expressions in REPL

[jcubic]

Examples of issues with the parser:

lips> "
lips>
lips> ''
;; => (quote #<eof>)

BiwaScheme have this code in REPL:

function unbalanced_parentheses(text_code) {
    try {
        (new BiwaScheme.Parser(text_code)).getObject(text_code);
    } catch(e) {
        if (e instanceof BiwaScheme.Parser.Unterminated) {
            return true;
        } else {
            return false;
        }
    }
    return false;
}