Fix the error message when a new benchmark is added without sharding info

tools/perf/core/perf_data_generator.py

 #!/usr/bin/env python
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # pylint: disable=too-many-lines
 
 """Script to generate chromium.perf.json and chromium.perf.fyi.json in
 the src/testing/buildbot directory and benchmark.csv in the src/tools/perf
 directory. Maintaining these files by hand is too unwieldy.
@@ skipping 656 matching lines @@
   for benchmark in benchmarks:
     if not ShouldBenchmarkBeScheduled(benchmark, tester_config['platform']):
       continue
 
     # First figure out swarming dimensions this test needs to be triggered on.
     # For each set of dimensions it is only triggered on one of the devices
     swarming_dimensions = []
     for dimension in tester_config['swarming_dimensions']:
       device = None
       sharding_map = benchmark_sharding_map.get(name, None)
-      if not sharding_map:
+      device = sharding_map.get(benchmark.Name(), None)
+      if device is None:
         raise ValueError('No sharding map for benchmark %r found. Please'
                          ' disable the benchmark with @Disabled(\'all\'), and'
                          ' file a bug with Speed>Benchmarks>Waterfall'
                          ' component and cc martiniss@ and nednguyen@ to'
                          ' execute the benchmark on the waterfall.' % (
-                             name))
+                             benchmark.Name()))
 
-      device = sharding_map.get(benchmark.Name(), None)
-
-      if device is None:
-        raise Exception('Device affinity for benchmark %s not found'
-          % benchmark.Name())
       swarming_dimensions.append(get_swarming_dimension(
           dimension, device))
 
     test = generate_telemetry_test(
       swarming_dimensions, benchmark.Name(), browser_name)
     isolated_scripts.append(test)
     # Now create another executable for this benchmark on the reference browser
     # if it is not blacklisted from running on the reference browser.
     if benchmark.Name() not in benchmark_ref_build_blacklist:
       reference_test = generate_telemetry_test(
@@ skipping 303 matching lines @@
       return 0
     else:
       print ('The perf JSON config files are not up-to-date. Please run %s '
              'without --validate-only flag to update the perf JSON '
              'configs and benchmark.csv.') % sys.argv[0]
       return 1
   else:
     update_all_tests([fyi_waterfall, waterfall])
     update_benchmark_csv()
   return 0