debug, fix panic with zero-duration recording

I had an assert that fired in this case, dating back to when I hadn't plumbed
Result returns through much of .mp4 construction. Now I have, so there's no
excuse in having an assert here. Change to an error return, and tweak it to
not fire in the zero-duration case.

Also fix a problem in the test harness; I hadn't finished converting it for
multi-recording tests, and it was returning the wrong recording.

Because of that, I seem to have stumbled across a related problem in which
asking for zero duration of a non-zero duration recording will return a
recording::Segment with no frames, which will cause panics because its
corresponding .mp4 slices are zero-length. I just adjusted the panic message
here; I'll follow up with changes to address that.

guide/install.md

@@ -171,6 +171,7 @@ been done for you. If not, Create
     Environment=TZ=:/etc/localtime
     Environment=MOONFIRE_FORMAT=google-systemd
     Environment=MOONFIRE_LOG=info
+    Environment=RUST_BACKTRACE=1
     Type=simple
     User=moonfire-nvr
     Nice=-20