Fix `bot summary` when URIs couldn't be read.

tools/gardening/lib/src/bot.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'buildbot_data.dart';
 import 'buildbot_structures.dart';
 import 'client.dart';
 import 'util.dart';
@@ skipping 20 matching lines @@
       buildUri = buildUri.prev();
       uris.add(buildUri);
     }
     return readResults(uris);
   }
 
   Future<BuildResult> readResult(BuildUri buildUri) {
     return _client.readResult(buildUri);
   }
 
+  /// Reads the build results of all given uris.
+  ///
+  /// Returns a list of the results. If a uri couldn't be read, then the entry
+  /// in the list is `null`.
   Future<List<BuildResult>> readResults(List<BuildUri> buildUris) async {
     var result = <BuildResult>[];
     int i = 0;
     const maxParallel = 20;
     while (i < buildUris.length) {
       var end = i + maxParallel;
       if (end > buildUris.length) end = buildUris.length;
       var parallelChunk = buildUris.sublist(i, end);
       log("Fetching ${end - i} uris in parallel");
-      result.addAll(await Future.wait(parallelChunk.map(_client.readResult)));
+      result.addAll(await Future.wait(parallelChunk.map((uri) {
+        var result = _client.readResult(uri);
+        if (result == null) {
+          log("Error while reading $uri");
+        }
+        return result;
+      })));
       i = end + 1;
     }
     return result;
   }
 
   /// Returns uris for the most recent build of all build groups.
   List<BuildUri> get mostRecentUris {
     List<BuildUri> result = [];
     for (BuildGroup group in buildGroups) {
       result.addAll(group.createUris(_client.mostRecentBuildNumber));
     }
     return result;
   }
 
   /// Closes the bot.
   void close() => _client.close();
 }