Return a valid error code when adding an invalid IP address.

Currently, passing an invalid IP address to interfaceAddAddress
and friends results in an invalid error message:

    12-02 13:00:42.675 interfaceAddAddress("testtap2355", "2001:db8::1/64", 64) -> ServiceSpecificException(-8, "InterfaceController error: Unknown error -8") <1.63ms>

This is due to confusion between getaddrinfo error codes and
errno values.

Test: new unit test added to IpClientIntegrationTest
Change-Id: Ifdaa4281a9bcf3998e3216472c5c1df0f5285214