2.0 Click casting - success/fail check, help!

[Aznamir]

So I have a button that inherits SecureActionButtonTemplate

I made 2 functions:

function PallyPower:ButtonPreClick(mousebutton)
  if (not InCombatLockdown()) then
    local classID = this:GetAttribute("classID");
    local spell = PallyPower:GetSpell(classID);
    local gspell = PallyPower_Greater .. spell;
    local unitid = PallyPower:GetUnitSmart(classID, mousebutton, spell);
    if not unitid then
        spell = nil;
        gspell = nil;
    end
    -- left click (do 15 minute buff)
    this:SetAttribute("type1", "spell");
    this:SetAttribute("unit1", unitid);
    this:SetAttribute("spell1", gspell);
    -- right click (do a 5 minute buff)
    this:SetAttribute("type2", "spell");
    this:SetAttribute("unit2", unitid);
    this:SetAttribute("spell2", spell);
  end
end

function PallyPower:ButtonPostClick(mousebutton)
  local classID = this:GetAttribute("classID");
  -- decisions, decisions...
  if (mousebutton == "LeftButton") then
    LastCast[classID] = 15 * 60;
  end
end

So on pre-click the mod find a nearby who is in range (according to IsSpellInRange() function) class member and assigns the spell attributes tot he button.

Problems:
1) If unitid or spell are nil (noone in range), the button does not do anything, and I need to catch the situation and not reset the timer. How to do it?

2) sometimes when people move out of range the click returns a red error message, but sometimes the cursor turns into a highligted hand, I guess expecting me to click on someone. How to prevent it?
I just don't want to cast anything if the result will be an obvious failure.

Could someone help?