Elder Sign Terror Effect Timing
Per the rulebook as written, the terror effect directly results from the failed task roll with the terror result showing.  It takes effect before a die is discarded due to the failed roll.

I would presume by that wording that they are trying to prevent exactly what you are describing - avoiding the terror by discarding the die.  I can't find any errata to clarify the situation.  There is clearly a gap in the timing, otherwise adding dice previously saved couldn't work and once a roll succeeds by that method the terror doesn't apply. Still, once a roll is clearly failed, the terror has to be done before you proceed.

The rules on focusing and assistance are clearer.  Although they save a result and remove a die from the pool, they do so only after a failed roll has already been dealt with.  Since spells can be played anytime on your own turn, this introduces a grey area.  Still, there is nothing in the rules that states a failure can be avoided by removing dice from it, and a terror is part of the failure.

Putting it a better way since I ran a few searches - you generated a terror on a failed roll.  Why would you not suffer an effect simply because you're saving the dice for later?  That just confirms you rolled it.

So my answer would be, unless you can by action change a failed roll into a successful roll, you would have to go through with the terror effect.