Make wait_for_status robust against multiple calls at different times.

This change removes the dependency between wait_for_status? and
download? by checking for the staged directory if there is the
equivalent of a "cache" miss from the downloader_dict.

I also clean up a previous assumption in the downlaoder.

BUG=chromium-os:27285
TEST=Unittests + download followed by wait_for_status with
same archive_url and different.  Got all expected results.
Also ran pylint on both devserver and downloader and fixed a cpl
bugs it caught.

Change-Id: I8c187cd64e40c12442de3e4be8fd4c12db7958f3
Reviewed-on: https://gerrit.chromium.org/gerrit/19263
Commit-Ready: Chris Sosa <sosa@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
diff --git a/devserver_util.py b/devserver_util.py
index c05ec41..fa92a97 100644
--- a/devserver_util.py
+++ b/devserver_util.py
@@ -151,7 +151,7 @@
   """
   build_dir = os.path.join(static_dir, tag)
   if not SafeSandboxAccess(static_dir, build_dir):
-    raise DevServerUtilError('Invaid tag "%s".' % tag)
+    raise DevServerUtilError('Invalid tag "%s".' % tag)
 
   try:
     os.makedirs(build_dir)