fix: allow for context-specific reader-facing error messsages

[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:

Builds on filters added in #1745 by allowing reader-facing error messages to be altered depending on the context of the original error thrown by the ESP's API.

This PR adds a special error message for the Mailchimp error that occurs when a contact attempts to resubscribe to a list they've previously unsubscribed from. In all other cases, the error shown will be a generic error: Sorry, an error has occurred. Please try again later or contact us for support.

Also standardizes the error code used across all ESP's when add_contact results in an error to: newspack_{esp}_api_error

How to test the changes in this Pull Request:

set up a contact for the compliance state

1. Check out this branch.
2. With Mailchimp enabled, sign up as a reader with a real email address for a test newsletter list/group/tag via any means on the site.
3. Send a live newsletter campaign to the list/group/tag that the reader will receive.
4. Once you receive the email in the reader's inbox, click the "Unsubscribe" link at the bottom and confirm.
5. In a new session, attempt to resubscribe to the same newsletter lists using the same reader email.
6. Confirm the following error is shown:

7. Temporarily force an error to occur in the first line of the Mailchimp class validate() method:

	public function validate( $result, $preferred_error = null, $payload = [] ) {
+		$result['status'] = 400;
+		$result['title']  = 'Test error';
+		$result['detail'] = 'Test error message';
		$default_error = $this->get_reader_error_message( $payload, $result );

8. In a new session, subscribe to a newsletter list with a fresh email address and confirm you get the generic error message:

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?

[codecov]

Codecov Report

Attention: Patch coverage is 53.12500% with 15 lines in your changes missing coverage. Please review.

Project coverage is 23.31%. Comparing base (69ed769) to head (3d928eb).
Report is 34 commits behind head on release.

Files with missing lines	Patch %	Lines
...ign/class-newspack-newsletters-active-campaign.php	0.00%	4 Missing ⚠️
...or/class-newspack-newsletters-campaign-monitor.php	0.00%	4 Missing ⚠️
...ct/class-newspack-newsletters-constant-contact.php	0.00%	4 Missing ⚠️
...mailchimp/class-newspack-newsletters-mailchimp.php	81.25%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             release    #1754      +/-   ##
=============================================
+ Coverage      23.29%   23.31%   +0.01%     
- Complexity      2710     2720      +10     
=============================================
  Files             49       49              
  Lines          10754    10853      +99     
=============================================
+ Hits            2505     2530      +25     
- Misses          8249     8323      +74     

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