The following error codes may occur when evaluating custom functions, as per Microsft docuemntation

ErrorCode enum value	Excel cell value	Description
divisionByZero	#DIV/0	The function is attempting to divide by zero.
invalidName	#NAME?	There is a typo in the function name. Note that this error is supported as a custom function input error, but not as a custom function output error.
invalidNumber	#NUM!	There is a problem with a number in the formula.
invalidReference	#REF!	The function refers to an invalid cell. Note that this error is supported as a custom function input error, but not as a custom function output error.
invalidValue	#VALUE!	A value in the formula is of the wrong type.
notAvailable	#N/A	The function or service isn't available.
nullReference	#NULL!	The ranges in the formula don't intersect.
spill range isn't blank	#SPILL!	#SPILL errors are returned when a formula returns multiple results, and Excel cannot return the results to the grid.