Add an explicitly output error message to the test bootstrap file

[johnbillion]

Previously: #55

If the server's display_errors directive is set to Off and the user tries to run their unit tests without first running bash bin/install-wp-tests.sh ..., they'll be greeted with blank output.

The uncaught exception is good if you have access to the error logs, or think to look there, but an error message which is explicitly output (via echo) would be even better.

[johnbillion]

Recommendation for the bootstrap template:

if ( ! file_exists( $_tests_dir . '/includes/functions.php' ) ) {
	echo "Could not find $_tests_dir/includes/functions.php, have you run bin/install-wp-tests.sh ?";
	exit( 1 );
}

[danielbachhuber]

👍