VideoPress: space storage limits not visible to user

#### Steps to reproduce the issue

0. Start from a site using a Jetpack Premium Plan.
1. Upload a lot of big videos, enough to go over the 13Go quota included in Jetpack Premium.

How do you know how much space you have left? What happens when you reach your quota? It turns out you reach it after only 3Go instead of 13G0, and the only way to figure that out is to check the response you get when trying to upload a new video:

```
{"error":"upload_error","message":"Not enough space to upload. 589,305 KB needed. Back"}
```

Related: #5986

Start by reproducing the VideoPress upload behavior described in the issue and inspect where the quota and upload error are surfaced to users. Determine how the Jetpack Premium storage limit is calculated and displayed, then verify that the visible limit matches the plan and that reaching it provides a clear user-facing message.

Written by the indexing model from the issue text.