throws exception for non-existing packages

[danrubel]

Minor thing, but probably should not throw exception when no package found...

$pub global run den install logger packageName: logger Unhandled exception: Uncaught Error: The null object does not have a getter 'versions'.

NoSuchMethodError: method not found: 'versions' Receiver: null Arguments: [] Stack Trace: #0 Object._noSuchMethod (dart:core-patch/object_patch.dart:42) #1 Object.noSuchMethod (dart:core-patch/object_patch.dart:45) #2 fetchPrimaryVersion. (package:den_api/src/util.dart:33) #3 _RootZone.runUnary (dart:async/zone.dart:1155) #4 _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:494) #5 _Future._propagateToListeners (dart:async/future_impl.dart:577) #6 _Future._completeWithValue (dart:async/future_impl.dart:368) #7 _Future._asyncComplete. (dart:async/future_impl.dart:422) #8 _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:41) #9 _asyncRunCallback (dart:async/schedule_microtask.dart:48) #10 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96) #11 _Timer._runTimers (dart:isolate-patch/timer_impl.dart:392) #12 _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:411) #13 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:142) #0 _rootHandleUncaughtError. (dart:async/zone.dart:886) #1 _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:41) #2 _asyncRunCallback (dart:async/schedule_microtask.dart:48) #3 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96) #4 _Timer._runTimers (dart:isolate-patch/timer_impl.dart:392) #5 _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:411) #6 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:142)