move docker_tls_args into weaveutil

and also give users a clue what went wrong when docker isn't running

.gitignore

@@ -42,7 +42,6 @@ prog/weavewait/weavewait
 prog/weavewait/weavewait_noop
 prog/weavewait/weavewait_nomcast
 prog/weaveutil/weaveutil
-prog/docker_tls_args/docker_tls_args
 prog/plugin/plugin
 testing/cover/cover
 testing/runner/runner
@@ -60,7 +59,6 @@ prog/weaveexec/weavewait_noop
 prog/weaveexec/weavewait_nomcast
 prog/weaveexec/sigproxy
 prog/weaveexec/weaveutil
-prog/weaveexec/docker_tls_args
 prog/weaveexec/docker*.tgz
 Vagrantfile.local
 test/tls/tls