Migrate pub away from throwing strings.

utils/pub/hosted_source.dart

Index: utils/pub/hosted_source.dart
diff --git a/utils/pub/hosted_source.dart b/utils/pub/hosted_source.dart
index 5a4106e29cac657b57ea2dafe8abcf349f93223c..a9e80cb0761cde4c79f4222c0210fcbf59405318 100644
--- a/utils/pub/hosted_source.dart
+++ b/utils/pub/hosted_source.dart
@@ -133,16 +133,19 @@ class HostedSource extends Source {
   void _throwFriendlyError(error, package, url) {
     if (error is PubHttpException &&
         error.response.statusCode == 404) {
-      throw 'Could not find package "$package" at $url.';
+      throw new UserFacingException('Could not find package "$package" at '
+          '$url.');
     }
 
     if (error is TimeoutException) {
-      throw 'Timed out trying to find package "$package" at $url.';
+      throw new UserFacingException('Timed out trying to find package '
+          '"$package" at $url.');
     }
 
     if (error is io.SocketIOException) {
-      throw 'Got socket error trying to find package "$package" at $url.\n'
-          '${error.osError}';
+      throw new UserFacingException('Got socket error trying to find package '
+              '"$package" at $url.\n'

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

Nit, but I would probably move the whole string to this line and try to get rid
of the line continuation.

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

This was reformatted anyway due to [fail].

+          '${error.osError}');
     }
 
     // Otherwise re-throw the original exception.