Re-opening because the original patch broke test cases that relied on
detectors being able to raise:

  ValueError
  pytz.UnknownTimeZoneError

I have a patch that fixes the test cases in the roundup trunk
[committed as 89d69a822e5c] but I am not happy with it. Emailed devel
list for ideas on how to better handle this.

I am considering changing the original patch, which captured all
Exceptions, to capturing: EnvironmentError (which include IOError and
OSError IIUC) and ArithmeticError and let the rest propagate up.

I wasn't able to find a canonical list of the exceptions that a
detector can raise that have to be propagated up into the parent, if I
could then the code could be structured to all these to pass up and
still capture all other exceptions.