nvmf: print error if transport name isn't found

Rather than failing silently, let the user know why the listen address
failed.

Change-Id: I41c2a51c6071ee739b282a1a39198a2887a73c4d
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

lib/nvmf/subsystem.c

@@ -275,6 +275,7 @@ spdk_nvmf_tgt_listen(const char *trname, const char *traddr, const char *trsvcid
 
 	transport = spdk_nvmf_transport_get(trname);
 	if (!transport) {
+		SPDK_ERRLOG("Unknown transport '%s'\n", trname);
 		return NULL;
 	}