Fix ajax request callbacks

[ilicfilip]

Context

We do AJAX requests from one function, to which we pass data, URL, success and fail callbacks.
In that function we're registering new callback on the http.onreadystatechange event, success callback was executing fine but fail callback was triggered on every state change - mainly on when http.readyState values were 2 & 3 (headers received & response downloading), which is wrong.

I have changed it so fail callback is triggered only when request is done, but response code is not 200.

I noticed this while working on new Onboarding flow, scanning posts was triggered twice and there were multiple 'Failed to scan posts. Retrying...' messages in the console, which looked suspicious.

Summary

This PR can be summarized in the following changelog entry:

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

[github-actions]

Test merged PR on Playground
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[jdevalk]

I'll let @aristath review too, but this looks good to me.