Handle unknown options better

[kwalrath]

This is a minor, fit-and-finish kind of thing, but it should be easy to fix when you have a little time. When you enter an unknown option, you get a stack trace (in addition to a usage message). You should instead just get the usage message.

$ gist generate --unknown-flag
Unhandled exception:
Uncaught Error: Could not find an option named "unknown-flag".

Usage: gist <command> [arguments]

Global options:
-h, --help    Print this usage information.

Available commands:
  generate   Generate gists from the current directory.
  help       Display help information for gist.

Run "gist help <command>" for more information about a command.
Stack Trace:
#0      CommandRunner.usageException (package:args/command_runner.dart:88)
#1      CommandRunner.parse (package:args/command_runner.dart:118)
#2      CommandRunner.run.<anonymous closure> (package:args/command_runner.dart:105)
#3      Future.Future.sync (dart:async/future.dart:168)
#4      CommandRunner.run (package:args/command_runner.dart:105)
#5      main (http://localhost:50006/gist.dart:14)
#6      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:259)
#7      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)

#0      _rootHandleUncaughtError.<anonymous closure> (dart:async/zone.dart:895)
#1      _microtaskLoop (dart:async/schedule_microtask.dart:43)
#2      _microtaskLoopEntry (dart:async/schedule_microtask.dart:52)
#3      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)
$ 

[kasperpeulen]

@kwalrath haha, thanks for the help, fixed now :D

[kwalrath]

Thanks!