Error: undefined caused by opening multiple notebooks simultaneously

Here's a new error that I saw for the first time when trying to open a notebook:


What I did that triggered this error: I went to http://data8.org/sp17/ and double-clicked on three links to three notebooks in rapid succession, causing each one to open in a separate tab. (I opened the lec16, lec17, and lec18 demos notebooks, i.e., Fri 2/24, .) This created three tabs with links to git-pull URLs. The first tab loaded the notebook. The next two tabs triggered the above error message.

This is very minor and low-impact. If I just re-click the link, everything works. But this makes me suspect there is something failing if a user tries to simultaneously do multiple pull/sync operations, and failing in a way that displays an unhelpful error message. So a small improvement could be to display a more helpful error message, or to write the system to wait a random amount of time and try again.