Print the failing command if subprocess.call() fails

Change-Id: Ib640b590e259a1a9bd44030ae13611cd359721e0

packager/app/test/packager_app.py

@@ -45,7 +45,8 @@ class PackagerApp(object):
     # Put single-quotes around each entry so that things like '$' signs in
     # segment templates won't be interpreted as shell variables.
     self.packaging_command_line = ' '.join(["'%s'" % entry for entry in cmd])
-    assert 0 == subprocess.call(cmd)
+    assert 0 == subprocess.call(cmd), ('%s returned non-0 status' %
+                                       self.packaging_command_line)
 
   def GetCommandLine(self):
     return self.packaging_command_line