USB Error after failed attempt at flashing
Answering my own question:  I fixed it, and I'm not sure how.

I did several things, and I did them several times. Can't tell which thing or combination of things worked, but here's what I tried:


Removed the sd card.
Connected the phone to the pc without booting the phone. This took it into Recovery mode, sometimes but not always, and I didn't figure out why sometimes it went into Recovery mode and sometimes it didn't. From Recovery mode, I selected and ran the Factory Restore option.
Removed the battery and left it out for several minutes. (I kind of think this was important.)


Eventually the USB error vanished, Odin discovered the phone, and I was able to reflash the stock ROM.