Print avahi error if avahi_entry_group_new

Publishing can be disabled in avahi daemon config.

Change-Id: I5338e530fbabf005eb9c3fd8a558092248c2f3ba

libweave/examples/ubuntu/avahi_client.cc

diff --git a/libweave/examples/ubuntu/avahi_client.cc b/libweave/examples/ubuntu/avahi_client.cc
index 11f16c7..424cee0 100644
--- a/libweave/examples/ubuntu/avahi_client.cc
+++ b/libweave/examples/ubuntu/avahi_client.cc
@@ -37,8 +37,7 @@
   avahi_threaded_poll_start(thread_pool_.get());
 
   group_.reset(avahi_entry_group_new(client_.get(), GroupCallback, nullptr));
-
-  CHECK(group_);
+  CHECK(group_) << avahi_strerror(avahi_client_errno(client_.get()));
 }
 
 MdnsImpl::~MdnsImpl() {