Source

File: wp-includes/wp-db.php

public function bail( $message, $error_code = '500' ) {
		if ( $this->show_errors ) {
			$error = '';

			if ( $this->use_mysqli ) {
				if ( $this->dbh instanceof mysqli ) {
					$error = mysqli_error( $this->dbh );
				} elseif ( mysqli_connect_errno() ) {
					$error = mysqli_connect_error();
				}
			} else {
				if ( is_resource( $this->dbh ) ) {
					$error = mysql_error( $this->dbh );
				} else {
					$error = mysql_error();
				}
			}

			if ( $error ) {
				$message = '<p><code>' . $error . "</code></p>\n" . $message;
			}

			wp_die( $message );
		} else {
			if ( class_exists( 'WP_Error', false ) ) {
				$this->error = new WP_Error( $error_code, $message );
			} else {
				$this->error = $message;
			}

			return false;
		}
	}