Show expected error message when docker_tls_args fails.
Show expected error message when docker_tls_args fails.

And also make the error message more meaningful.

Fixes #1843.

tomwilkie/weave,rade/weave,weaveworks/weave,paulbellamy/weave,n054/weave,dpw/weave,weaveworks/weave,rade/weave,paulbellamy/weave,paulbellamy/weave,weaveworks/weave,tomwilkie/weave,n054/weave,dpw/weave,n054/weave,tomwilkie/weave,rade/weave,n054/weave,dpw/weave,rade/weave,tomwilkie/weave,weaveworks/weave,dpw/weave,rade/weave,tomwilkie/weave,paulbellamy/weave,n054/weave,dpw/weave