How to determine why a spellcast failed

[Lodran]

I'm working on a plugin that implements a blacklist functionality when casting spells.

Unfortunately, right now it blacklists units for reasons that it shouldn't - for example, when you are stunned, silenced or out of mana.

So, when my SPELLCAST_FAILED event is called, how can I determine why the spell failed, so that I can avoid blacklisting players for stupid reasons?