NS_Emulation: Fail if an unsupported NS provider is chosen

This took me quite some time: Tried to use NS_PROVIDER_FR, but the
code was compiled without support for it.  It just failed silently
without printing any error or ever sending any message on the FR link.

Change-Id: I96475127a2079830b3456a8e288adf4c6c908887
1 file changed