Migrate pub away from throwing strings.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index b162859cf49385bab3f929c8ef30496491664a69..761087103ab27b1c483b8fb8c69b6f3b0977b79f 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -87,8 +87,8 @@ class Entrypoint {
       } else {
         return id.source.install(id, packageDir).then((found) {
           if (found) return null;
-          // TODO(nweiz): More robust error-handling.
-          throw 'Package ${id.name} not found in source "${id.source.name}".';
+          throw new UserFacingException('Package ${id.name} not found in '

[Bob Nystrom, 2013/04/18 18:18:31]

That name is a bit strange. How about "PubFailure"?

Also, how do you feel about a helper function that does this, like:

fail(String message) => throw new ...

That might get rid of some of the annoying wrapped strings.

[nweiz, 2013/04/18 18:37:24]

"PubFailure" sounds very generic to me. Is an unexpected error not also pub
failing?
Done.

[Bob Nystrom, 2013/04/18 20:02:13]

I guess so, but I'd consider that more of an underlying failure. Maybe
ApplicationException?

[nweiz, 2013/04/18 20:12:01]

Done.

+              'source "${id.source.name}".');
         });
       }
     }).then((_) => id.resolved);