When the collector is down, ooniprobe hangs

When a collector is not reachable (host resolution fails) ooniprobe will hang.

This is the backtrace:

[D] Adding report entry task <ooni.tasks.ReportEntry object at 0x104196f50>
[D] Writing report with YAML reporter
[D] Adding report entry task <ooni.tasks.ReportEntry object at 0x1038ec590>
Traceback (most recent call last):
  File "/XXXXXXXXXXX/reporter.py", line 339, in createReport
    bodyProducer=bodyProducer)
HostUnreachable
[!] Failed to open <ooni.reporter.OONIBReporter object at 0x104196f90> reporter, giving up...
[!] Reporter <ooni.reporter.OONIBReporter object at 0x104196f90> failed, removing from report...