Add custom error message when user has exceeded capacity

[MauAraujo]

Feature Description
PR #7375 added a check and configurable parameter to the backend so that users can't start more mandelboxes than allowed. So, if the limit is exceeded, the backend responds with a 503 error and includes an error field in the response with the error code USER_ALREADY_ACTIVE

The Problem It Fixes
Currently, if the limit is exceeded, we display a generic error message that says "Our servers are at capacity".

We should display a more relevant error that indicates to the user that they have reached the limit of mandelboxes for that session.

Note: a mandelbox is started per profile, so it makes more sense to word the message as a profile limit.

Potential Implementation(s)
The extension already has code to handle error codes received in the response, so it's just a matter of adding another case and displaying the new message.