What formula should be used? help?

[xtrax]

Currently, i've done this
=IF(H3>=101,"ERROR",IF(H3>=70,"A",IF(H3>=60,"B",IF(H3>=50,"C",IF(H3>=40,"D",IF(H3<=39,"FAIL","ERROR"))))))
which works, its a calculation for a full module grade.

but the problem comes if I calculate 1/2 modules where i have to add both marks together and get a grade, but If i was to type anything else other than 1-100, I would want it to show the word "error" help!

[xtrax]

MT105A Mathematics 1 (half unit) A 0
MT105A Mathematics 1 (half unit) A

when i SUM(the 2 A), I'll get 0, isit possible to get an error or something to notify its wrong