Allow ENUM query functions to report lookup errors

The ENUM dialplan functions do not report DNS query errors properly. It is
useful to differentiate between failed query (e.g. non-existent domain) vs. no
data records of the appropriate type. This is required to make overlapped
dialing work.

(closes issue ASTERISK-13769)
Review: https://reviewboard.asterisk.org/r/1355/
Patch-by: Timo Teras


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@331201 65c4cc65-6c06-0410-ace0-fbb531ad65f3