BtrvGetGreater goes to the next larger record of the file based on the
key number that you pass and reads the record greater than Value into
DataVar. The type of Value passed has to match the type of data for
which the key is defined. If the key is a string then Value is a string.
If BtrvResult is zero, the data was found. If BtrvGetPrev is called
after this, the next sequential record based on the key is read.
6 - Invalid Key Number