MySQL documentation on error handling is peculiarly hard to find unless you know what you are looking for. Here's where it is for MySQL 5.6:

DECLARE ... HANDLER: http://dev.mysql.com/doc/refman/5.6/en/declare-handler.html
SIGNAL: http://dev.mysql.com/doc/refman/5.6/en/signal.html
RESIGNAL: http://dev.mysql.com/doc/refman/5.6/en/resignal.html
GET DIAGNOSTICS: http://dev.mysql.com/doc/refman/5.6/en/get-diagnostics.html