Handle AccountNotFoundError case specially when quota lookup fails

[sentry]

Currently when we fail to lookup quotas via /api/v1/subscription/plan/info, we treat all lookups failure the same in thunderbird_accounts/subscription/views.py around line 280: We log at the error level (which notifies Sentry) and we return a generic failure message to the user.

There's one known case that we should handle differently which would address a common Sentry error and provide a clearer error message to users:

The app may throw the specific AccountNotFoundError. First, it would be useful to research how this state arises. Maybe a user logged in but in they are offboarding, so the Stalwart account is gone but their session still there?

Assuming we find that this is a state we expect to happen sometimes, then we should handle it different:

• Log at the warning level instead of error
• Update the JSON response with a more specific error: "User not found". Consider status=404 instead of 500 in this case, since it's not a server error.

Sentry Issue: THUNDERBIRD-ACCOUNTS-7J

Error getting used quota: AccountNotFoundError: [email]

[MooseTheRebel]

Hello 👋

I'd like to work on this issue. I think I can draft a fix (with some assumptions).

That said, I do have some clarifying questions:

1. Is there any additional data that can be shared about why AccountNotFoundError fires here — e.g., is it specifically account offboarding, or could it also occur for other reasons? Is there anything from the Sentry breadcrumbs/user state for a few instances of THUNDERBIRD-ACCOUNTS-7J that you're able to share?
2. Should the response be status=404 with {'success': False, 'error': 'User not found'} exactly, or does the frontend key off a specific error code/value rather than displaying the string as-is?
3. There are a few other places that call get_account() with a generic except Exception — should those get the same fix, or is this issue just about subscription/views.py?