Some small Error Tracker improvements

[Florent 'Skia' Jacquet]

Hello there, users of https://errors.ubuntu.com,

I recall to have seen a few reports that the Error Tracker failed to load the 
top crashes recently. This is now fixed, and since I couldn't find those reports 
anymore after coming back from PTO, I'm taking the occasion to write here.

Along the way, I've cowboyed a quick change: the number of loaded crashes is now 
halved to 50 instead of 100. This makes the table load way faster, while still 
providing a decent amount of data to work with. Please let me know if you'd like 
to see that change reverted, or if on the contrary I can go even lower.

Also keep in mind that for now, the Error Tracker's web part is still in its 
Bionicky Python 2 juice, so I'm still not confident rolling out changes bigger 
than cowboying hardcoded values like this 100 turned to 50.

Reports of things breaking on the Error Tracker are still much appreciated, also 
because it gives me a ~~better~~ view of the number, and what kind of 
people/team are making use of it.

Happy crash fixing!
Skia