Shows an error message if the server returned invalid json during a Sync

[dsawardekar]

Description of the Change

This PR improves error handling of the Dashboard Sync by displaying an error message when the admin-ajax calls return invalid JSON data.

This is what the Sync UI looks like without this fix on the develop branch, when the server returns an invalid JSON response.

And this is what the Sync UI looks on this feature branch with the same invalid JSON response.

To test this PR I created an mu-plugin with the following code. This can also be seen by introducing other output into admin-ajax calls like PHP Warnings, incorrectly output styles, etc.

<?php

add_action( 'admin_init', function() {
	if ( ! ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {
		return; 
	}

	throw new Exception( 'Exception during Admin AJAX' );
} );

Closes #2301

Possible Drawbacks

NA

Verification Process

• Create an mu-plugin and copy-paste the above admin_init code into it.
• Run the Sync from the Dashboard
• You should see an error message that the Sync failed.
• The logs tab should also show an invalid json error message.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed: Display and error message if syncing failed due to invalid JSON response from the server.

Credits

Props @dsawardekar