Swallow unhelpful IOErrors printed by queue feeder threads

In certain tests, where workers die in unexpected places, IOErrors will be printed for queue feeding. It doesn't seem we are handling these edge cases incorrectly, so for now we silence the exceptions (as to not be distracted by them and possibly overlook other ones).

However (as possibly obvious by what I wrote), I'm not comfortable doing this for non-testing cases (i.e., always muting those errors). They seem to only come up in exceptional cases, which hopefully indicate errors that users of vimap will handle (i.e. catch the exception in the worker routine, or fix the associated bug).