Retry registerQueue

[gnprice]

This is essential when we're re-registering (as we do since #184) after an event queue expired. Currently, if that re-register fails, the whole event-polling loop stops so that #184 and #185 are effectively defeated.

This appears to be the cause (on at least one occasion) of the polling failure mentioned in #555. See chat thread where we debugged it with the help of nginx logs.