cells: Catch UnresolvedAddressException

We have observed the following error in our logs:

13 Jan 2014 13:26:20 (lm) [] Uncaught exception in thread TunnelConnector
java.nio.channels.UnresolvedAddressException: null
        at sun.nio.ch.Net.checkAddress(Net.java:127) ~[na:1.7.0_45]
        at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:640) ~[na:1.7.0_45]
        at java.nio.channels.SocketChannel.open(SocketChannel.java:184) ~[na:1.7.0_45]
        at dmg.cells.network.LocationManagerConnector.connect(LocationManagerConnector.java:103) ~[cells-2.7.4.jar:2.7.4]
        at dmg.cells.network.LocationManagerConnector.run(LocationManagerConnector.java:153) ~[cells-2.7.4.jar:2.7.4]
        at dmg.cells.nucleus.CellNucleus$1.run(CellNucleus.java:674) ~[cells-2.7.4.jar:2.7.4]
        at java.lang.Thread.run(Thread.java:744) ~[na:1.7.0_45]

This patch fixes this problem by translating the exception to an
IOException with additional information about the address that could
not be resovled.

Target: trunk
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
(cherry picked from commit 50084af73b3b5ead96b9e675fd8c723f09c60d56)
Merge pull request #378 from gbehrmann/fix/2.6/rb6393

cells: Catch UnresolvedAddressException