Improve error message in subprocess2.py.

subprocess2.py

 # coding=utf8
 # Copyright (c) 2012 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.
 """Collection of subprocess wrapper functions.
 
 In theory you shouldn't need anything else in subprocess, or this module failed.
 """
 
 import cStringIO
@@ skipping 229 matching lines @@
         # Convert fork() emulation failure into a CygwinRebaseError().
         raise CygwinRebaseError(
             e.errno,
             args,
             kwargs.get('cwd'),
             None,
             'Visit '
             'http://code.google.com/p/chromium/wiki/CygwinDllRemappingFailure '
             'to learn how to fix this error; you need to rebase your cygwin '
             'dlls')
-      # Popen() can throw OSError when cwd or args[0] doesn't exist. Let it go
-      # through
-      raise
+      # Popen() can throw OSError when cwd or args[0] doesn't exist.
+      raise OSError('%s or %s probably doesn\'t exist' %
+                    (kwargs.get('cwd'), args[0]))
 
   def _tee_threads(self, input):  # pylint: disable=W0622
     """Does I/O for a process's pipes using threads.
 
     It's the simplest and slowest implementation. Expect very slow behavior.
 
     If there is a callback and it doesn't keep up with the calls, the timeout
     effectiveness will be delayed accordingly.
     """
     # Queue of either of <threadname> when done or (<threadname>, data).  In
@@ skipping 242 matching lines @@
 
   - Throws if return code is not 0.
   - Works even prior to python 2.7.
   - Blocks stdin by default if not specified since no output will be visible.
   - As per doc, "The stdout argument is not allowed as it is used internally."
   """
   kwargs.setdefault('stdin', VOID)
   if 'stdout' in kwargs:
     raise ValueError('stdout argument not allowed, it will be overridden.')
   return check_call_out(args, stdout=PIPE, **kwargs)[0]