relay_command_to_string() doesn't know about EXTEND2 / EXTENDED2
Apparently we left EXTEND2 and EXTENDED2 out of this function's big switch statement when we introduced these relay commands.
We could add the two lines in -- that would be easy, and somebody should.
The slightly more complicated question: is there a way to rephrase this function to future-proof it better against the next time we add a new relay command?