Question : How do I replace #N/A if it is returned in a formula? (not Vlookup)

I have a formula containing two if statements, but some of the values are #N/A.  How do I change that to return text that says "Data Not Available" instead of #N/A?

=IF(A20890>500000,"Channel III",IF(A20890<=500000,"Channel II"))

Answer : How do I replace #N/A if it is returned in a formula? (not Vlookup)

Or shorter:
=IF(ISNA(A20890),"Data not available",IF(A20890>500000,"Channel III","Channel II"))