Fix Bug 909930 ensure that crash_trends does not cause 500 errors when no Nightly

@peterbe r?

The best way to test this, besides running the tests ;), would be to test against the products that was returning the 500 errors mention in bug 909930.

This fixes the following bugs:
https://bugzilla.mozilla.org/show_bug.cgi?id=909930
https://bugzilla.mozilla.org/show_bug.cgi?id=887536

Implements the work around for:
https://bugzilla.mozilla.org/show_bug.cgi?id=915162

And fixes point 1 of:
https://bugzilla.mozilla.org/show_bug.cgi?id=914933