validate platforms and applications for update counts (1014752)

fixes [bug 1014752](https://bugzilla.mozilla.org/show_bug.cgi?id=1014752)

Implement some validation at our level (applications and platforms).
[The rest](https://bugzilla.mozilla.org/show_bug.cgi?id=1014752#c95) needs to
be done at the hive query level I think.
The mode is in the wrong parenthesis (it should be in `os.makedirs`, not in `os.path.join`)