BtrvGetLess goes to the next smaller record of the file based on the key
number that you pass and reads the record less 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