(The REM statements are simply comments and don't need to be included in the batch file)
The SET command lets you use expressions when you use the /A switch. So I set an environment variable "errlev" to a bitwise AND with the ERRORLEVEL value.
Robocopy's exit codes use a bitmap/bitmask to specify its exit status. The bits for 1, 2, and 4 do not indicate any kind of failure, but 8 and 16 do. So by adding 16 + 8 to get 24, and doing a bitwise AND, I suppress any of the other bits that might be set, and allow either or both of the error bits to pass.
The next step is to use the EXIT command with the /B switch to set a new ERRORLEVEL value, using the "errlev" variable. This will now return zero (unless Robocopy had real errors) and allow your SQL job step to report success.
This technique should also work for other command-line utilities. The only issues I've found is that it requires the commands to be part of a batch file, so if you use Robocopy directly in your SQL job step you'd need to place it in a batch. If you also have multiple Robocopy calls, you'll need to place the SET/A command ONLY after the last one. You'd therefore lose any errors from previous calls, unless you use multiple "errlev" variables and AND them together. (I'll leave this as an exercise for the reader)
The SET/A syntax also permits other kinds of expressions to be calculated. You can get a full list by running "SET /?" on a command prompt.