Slave fails with Abort stacktrace when DNS cannot resolve hostname

If the DNS cannot resolve the hostname-to-IP for a slave node, we correctly return an Error object, but we then fail with a segfault.

This code adds a more user-friendly message and exits normally (with an EXIT_FAILURE code).

For example, forcing net::getIp() to always return an Error, now causes the slave to exit like this:

$ ./bin/mesos-slave.sh --master=10.10.1.121:5405
WARNING: Logging before InitGoogleLogging() is written to STDERR
E0630 11:31:45.777465 1944417024 process.cpp:899] Could not obtain the IP address for stratos.local; the DNS service may not be able to resolve it: >>> Marco was here!!!

$ echo $?
1