Don't cache exceptions nor WP_Error objects

[stevegrunwell]

We should do our best to avoid caching WP_Error objects or PHP exceptions, as these are typically not things we want to cache. Explicitly checking for WP_Error objects (via is_wp_error() also enables developers to short-circuit the cache, if necessary.

For example, assume we have the following function:

remember_transient( 'some-cache-key', function () {
  $response = wp_remote_get( 'https://example.com' );

  // Something went wrong with the request.
  if ( is_wp_error( $response ) ) {
    return $response;
  } elseif ( 200 !== wp_remote_retrieve_response_code( $response ) ) {
    return new WP_Error( 'api-error', 'Received a non-200 status code.' );
  }

  return wp_remote_retrieve_body( $response );
}, DAY_IN_SECONDS );

If the response failed or returned a non-200 status code, that value could be cached for a day; temporary latency, a now-fixed bug, or any other number of reasons could result in stale errors being returned to users.

With this PR, the returned value is run through is_wp_error() before being stored in the object cache, transients, or site transients.

[coveralls]

Pull Request Test Coverage Report for Build 22

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 20:	0.0%
Covered Lines:	38
Relevant Lines:	38

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💛 - Coveralls