reverse_dns function needs error checking

[deadbits]

reverse_dns will throw herror "Unknown host" exceptions on IP addresses that don't have a PTR record.

>>> import utilitybelt as ub
>>> ub.reverse_dns('1.54.140.138')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "<input>", line 3, in reverse_dns
herror: [Errno 1] Unknown host
>>> print "why python gods why!"
why python gods why!

[sroberts]

Yeah, that's probably a fairly good point.

[deadbits]

I can make a PR to patch this later today. Just wanted to have the issue in for reference when I do :)

[krmaxwell]

:+1: