Migrate pub away from throwing strings.

utils/pub/http.dart

Index: utils/pub/http.dart
diff --git a/utils/pub/http.dart b/utils/pub/http.dart
index 8df0575fe0a8084d14c41e73e954945d7484cb50..454d7ccd128ee48d0149e6f718c53f2e41e14eeb 100644
--- a/utils/pub/http.dart
+++ b/utils/pub/http.dart
@@ -69,10 +69,11 @@ class PubHttpClient extends http.BaseClient {
             error.osError.errorCode == -5 ||
             error.osError.errorCode == 11001 ||
             error.osError.errorCode == 11004) {
-          throw 'Could not resolve URL "${request.url.origin}".';
+          throw new UserFacingException('Could not resolve URL '
+              '"${request.url.origin}".');
         } else if (error.osError.errorCode == -12276) {
-          throw 'Unable to validate SSL certificate for '
-              '"${request.url.origin}".';
+          throw new UserFacingException('Unable to validate SSL certificate '
+              'for "${request.url.origin}".');
         }
       }
       throw error;
@@ -188,7 +189,7 @@ Map parseJsonResponse(http.Response response) {
 
 /// Throws an error describing an invalid response from the server.
 void invalidServerResponse(http.Response response) {
-  throw 'Invalid server response:\n${response.body}';
+  throw new UserFacingException('Invalid server response:\n${response.body}');
 }
 
 /// Exception thrown when an HTTP operation fails.