Do not consider missing command as internal failure.

client/isolateserver.py

 #!/usr/bin/env python
 # Copyright 2013 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
 """Archives a set of files or directories to an Isolate Server."""
 
 __version__ = '0.4.5'
 
 import base64
@@ skipping 92 matching lines @@
 # A class to use to communicate with the server by default. Can be changed by
 # 'set_storage_api_class'. Default is IsolateServer.
 _storage_api_cls = None
 
 
 class Error(Exception):
   """Generic runtime error."""
   pass
 
 
+class IsolatedErrorNoCommand(isolated_format.IsolatedError):
+  """Signals an early abort due to lack of command specified."""
+  pass
+
+
 class Aborted(Error):
   """Operation aborted."""
   pass
 
 
 def stream_read(stream, chunk_size):
   """Reads chunks from |stream| and yields them."""
   while True:
     data = stream.read(chunk_size)
     if not data:
@@ skipping 1699 matching lines @@
         except IOError:
           raise isolated_format.MappingError(
               '%s doesn\'t seem to be a valid file. Did you intent to pass a '
               'valid hash?' % isolated_hash)
 
       # Load all *.isolated and start loading rest of the files.
       bundle.fetch(fetch_queue, isolated_hash, algo)
       if require_command and not bundle.command:
         # TODO(vadimsh): All fetch operations are already enqueue and there's no
         # easy way to cancel them.
-        raise isolated_format.IsolatedError('No command to run')
+        raise IsolatedErrorNoCommand()
 
     with tools.Profiler('GetRest'):
       # Create file system hierarchy.
       if not fs.isdir(outdir):
         fs.makedirs(outdir)
       create_directories(outdir, bundle.files)
       create_symlinks(outdir, bundle.files.iteritems())
 
       # Ensure working directory exists.
       cwd = os.path.normpath(os.path.join(outdir, bundle.relative_cwd))
@@ skipping 340 matching lines @@
 def main(args):
   dispatcher = subcommand.CommandDispatcher(__name__)
   return dispatcher.execute(OptionParserIsolateServer(), args)
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   tools.disable_buffering()
   colorama.init()
   sys.exit(main(sys.argv[1:]))