Press This: unhelpful error message on post save failure.

[stephdau]

We use a generic error message when we fail to save the post, which can be due something as simple as not having post title and content: ​https://cloudup.com/c9uxsiqQOIU

if ( is_wp_error( $updated ) || intval( $updated ) < 1 ) {
	wp_send_json_error( array( 'errorMessage' => __( 'Error while saving the post. Please try again later.' ) ) );
} else {

We should instead use $updated->get_error_message() to let the user know exactly what happened: ​https://cloudup.com/cT5H53fr5Ic

See attached patch.

Originally reported here: ​https://wordpress.slack.com/archives/core-pressthis/p1427516975000306

[SergeyBiryukov]

In 31930:

Press This: Use the error message returned by wp_update_post() instead of a generic one.

props stephdau.
fixes #31808.