don't fail `weave launch` when plugin is disabled
don't fail `weave launch` when plugin is disabled

Fixes #1851.

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