[My store] Fix error banner so it appears when stats data can't be loaded

[rachelmcr]

Closes: #10259

Description

The error banner (to explain there is a problem loading data) was not showing up on the My store dashboard. This PR fixes 3 related issues:

1. The check for sync errors was happening before the syncing was complete (and before any sync error was set).
   • Fix: In StoreStatsAndTopPerformersViewController there are several dispatch groups used for syncing (group, periodGroup, and periodStoreStatsGroup). We now call group.leave() last to make sure all the work for the time period is complete (including setting any period sync error) before checking for and handling sync errors for the dashboard as a whole.
2. We were attempting to fetch WPCom stats (visit and summary stats) even for sites not connected to WPCom, causing sync errors.
   • Fix: Now in DashboardViewModel we check if the store is authenticated with WPCom before dispatching those actions, to avoid extra API calls and unnecessary sync errors.
3. The pull to refresh mechanism has changed since the error banner was first introduced, and the error banner wasn't removed if the dashboard was refreshed after it appeared.
   • Fix: Now in DashboardViewController we hide the error banner optimistically on pull to refresh.

Testing instructions

1. Build and run the app.
2. Disable your internet connection and pull to refresh the dashboard (to trigger an error).
3. Confirm the error banner appears at the top of the screen (under the site title).
4. Enable your internet connection and pull to refresh again.
5. Confirm the error banner is removed and the dashboard loads as expected.

You can test using a store with or without Jetpack, to confirm the expected stats load in both cases.

Screenshots

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-07-19.at.14.10.56.mp4

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr10262-519ae01 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[jaclync]

Nice catch on the issue, works as expected

[jaclync]

nit: maybe it's worth making a comment so that we don't accidentally reorder it?

[rachelmcr]

good thought!