Correct flags parsing and report errors
Correct flags parsing and report errors

 - don't supply the executable name to `flagset.Parse(...)` (it was
   probably being treated as a non-flag arg, which we don't check for)

 - if flags fail to parse, instead of just exiting, report the error,
   print the usage, _then_ exit, so the term or logs will indicate why
   it exited.

weaveworks/flux,weaveworks/flux