Performance indicators - bail whenever an internal API request fails.

[jeffstieler]

Fixes #2288

When using rest_do_request() internally, add handling for cases where a WP_Error is returned.

Detailed test instructions:

Use the following snippet to force the error condition:

add_filter( 'rest_pre_dispatch', function( $pre, $server, $request ) {
        if (
            'GET' === $request->get_method() &&
            '/wc/v4/reports' === $request->get_route()
        ) {
            return new WP_Error( 'bam', 'pow' );
        }

        return $pre;
}, 10, 3 );

• On master, verify the error shows up in the logs when visiting the Dashboard
• Check out this branch
• Verify the error no longer occurs in log when visiting the Dashboard (Performance Indicators section will be empty though)

Changelog Note:

Fix: handle internal rest_do_request() error conditions.

[psealock]

LGTM, testing as advertised.