function makeVisible(txt) {
  document.getElementById('error_alert').style.display = 'block';
  document.getElementById(txt).style.display = 'block';
}

function hideDiv() {
   document.getElementById('error_alert').style.display = "none";   
   var lista = document.getElementById("error_alert").getElementsByTagName("div"); 
   for (var i=0; i<lista.length; i++){
     lista[i].style.display = "none";
   }
}

<!-- ValidaŤ‹o form Contacto -->

function contactFormCheck(form){

  hideDiv();
  
  var ok=true;
  
  if (form.nome.value.length <= 2) {
    makeVisible('erro_nome');
    ok=false;
  } 
  
  if (!(/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(form.email.value))) {
    makeVisible('erro_email');
    ok=false;
  }
  
  if (form.mensagem.value.length == 0) {
    makeVisible('erro_msg');
    ok=false;
  }

return(ok);
}

<!-- ValidaŤ‹o form Reserva -->

function reservaFormCheck(form){

  hideDiv();
  
  var ok=true;
  
  if (form.nome.value.length <= 2) {
    makeVisible('erro_nome');
    ok=false;
  } 
  
  if (!(/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(form.email.value))) {
    makeVisible('erro_email');
    ok=false;
  }
  
  if (form.tlf.value.length <= 5) {
    makeVisible('erro_tlf');
    ok=false;
  }

return(ok);
}