You can accomplish this by nesting functions. In this case
use the conditional function IF and the information
function ISERROR.
For example: Given that "CAL" is the table being looked
for and "A5" is the cell containing the lookup reference
the following formula will return an empty set if the
value in "A5" is not found.
=IF(ISERROR(VLOOKUP(A5,CAL,2,FALSE)),"",VLOOKUP
(A5,CAL,2,FALSE))
Note that the parenthesis returns an empty set. If you
would prefer a zero or some other fixed value, like "Value
not found" insert that in the parenthesis.