fix: fetching and display of sync errors after saving

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes a race condition with concurrent fetch requests in the Redux store that can prevent sync errors from being displayed, and improves the sync error messaging, especially for Mailchimp, which often returns errors in an additional errors array instead of in a single error message.

How to test the changes in this Pull Request:

1. On trunk, in a test site connected to Mailchimp, create a new newsletter.
2. Set the "Sender Name" to something with invalid characters, such as $ Invalid @ Sender ! Name #
3. Open the devtools console > Network tab and filter XHR requests by newsletters/v1. Save the newsletter as a draft and observe that after the save completes, a post-mjml POST request is fired, followed by a retrieve GET request, but possibly no sync-error request (because the fetchNewsletterData and fetchSyncErrors dispatchers are fired at the same time, the async response from fetchNewsletterData will overwrite the fetchSyncErrors request in the Redux store). Also observe that no error message is shown in the editor after this sequence of requests.
4. Check out this branch, repeat steps 2-3 and this time confirm that the save request is followed by a post-mjml POST request, a retrieve request, AND a sync-error request, like so:

5. Also confirm that a post-sync error message is shown in the editor with details of why the sync failed:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 20.86%. Comparing base (0ae5cc5) to head (bfc92d0).

Files with missing lines	Patch %	Lines
...mailchimp/class-newspack-newsletters-mailchimp.php	0.00%	3 Missing ⚠️
...ign/class-newspack-newsletters-active-campaign.php	0.00%	1 Missing ⚠️
...ct/class-newspack-newsletters-constant-contact.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #1670   +/-   ##
=========================================
  Coverage     20.85%   20.86%           
  Complexity     2658     2658           
=========================================
  Files            48       48           
  Lines          9776     9774    -2     
=========================================
  Hits           2039     2039           
+ Misses         7737     7735    -2     

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