Use a subprocess wrapper that will throw if a command returns a non-zero result code

utils.py

 # coding: utf-8
 
 # This Source Code is subject to the terms of the Mozilla Public License
 # version 2.0 (the "License"). You can obtain a copy of the License at
 # http://mozilla.org/MPL/2.0/.
 
-import sys, os, urllib, zipfile
+import sys, os, urllib, zipfile, subprocess
 from StringIO import StringIO
 
 def ensureJSShell():
   baseDir = os.path.dirname(__file__)
   shell_dir = os.path.join(baseDir, 'mozilla')
   if not os.path.exists(shell_dir):
     os.makedirs(shell_dir)
   if sys.platform == 'win32':
     path = os.path.join(shell_dir, 'js.exe')
   else:
@@ skipping 17 matching lines @@
 
   if not os.path.exists(path):
     raise Exception('Downloaded package didn\'t contain JS shell executable')
 
   try:
     os.chmod(path, 0700)
   except:
     pass
 
   return path
+
+STREAMS_NONE = 0
+STREAMS_STDOUT = 1
+STREAMS_STDERR = 2
+STREAMS_BOTH = 3
+def run(command, streams, **kwargs):
+  """
+    This function runs a command and returns the selected streams as result.
+    streams parameter indicates which streams need to be redirected and
+    returned, it can be either STREAMS_NONE, STREAMS_STDOUT, STREAMS_STDERR or
+    STREAMS_BOTH. An exception is thrown if the command cannot be executed or if
+    it returns a non-zero result.
+  """
+
+  if streams == STREAMS_STDOUT or streams == STREAMS_BOTH:
+    kwargs["stdout"] = subprocess.PIPE
+  if streams == STREAMS_STDERR or streams == STREAMS_BOTH:
+    kwargs["stderr"] = subprocess.PIPE
+  process = subprocess.Popen(command, **kwargs)
+  stdout, stderr = process.communicate()
+  if process.returncode != 0:
+    raise IOError("Subprocess didn't return a success code")
+  if streams == STREAMS_NONE:
+    return None
+  elif streams == STREAMS_STDOUT:
+    return stdout
+  elif streams == STREAMS_STDERR:
+    return stderr
+  elif streams == STREAMS_BOTH:
+    return (stdout, stderr)
+  else:
+    raise Exception("Invalid value for the streams parameter")