Migrate pub away from throwing strings.

utils/pub/command_lish.dart

Index: utils/pub/command_lish.dart
diff --git a/utils/pub/command_lish.dart b/utils/pub/command_lish.dart
index 8fa94df4c43a21c6474a0f94f2fe901a202315c9..1f767c9c94adc71d5a284103eb8b081e5c7a4e38 100644
--- a/utils/pub/command_lish.dart
+++ b/utils/pub/command_lish.dart
@@ -90,7 +90,7 @@ class LishCommand extends PubCommand {
         // TODO(nweiz): the response may have XML-formatted information about
         // the error. Try to parse that out once we have an easily-accessible
         // XML parser.
-        throw 'Failed to upload the package.';
+        throw new Exception('Failed to upload the package.');
       } else if (urisEqual(Uri.parse(url.origin), Uri.parse(server.origin))) {
         handleJsonError(error.response);
       } else {