Retry `fetch` requests

[baltpeter]

We often get error reports due to temporary network failures (like #659). To avoid those, and provide a better user experience, we should retry failed requests a couple of times.

[aerabi]

I can try to fix it. Where should I start looking?

[baltpeter]

@aerabi I've assigned you.

There isn't really any specific place where this could be fixed. You'd need to look through all instances in the code where we are using fetch(). And you should probably introduce a wrapper with retryability and replace those instances with that where it makes sense.

[aerabi]

I couldn't manage to install the project on my machine. I'll unassign myself to let other people contribute.