Add new ECHO mode 'errors' that displays only failed commands in psql.

When the psql variable ECHO is set to 'erros', only failed SQL commands
are printed to standard error output. Also this patch adds -b option into psql.
This is equivalent to setting the variable ECHO to 'errors'.

Pavel Stehule, reviewed by Fabrízio de Royes Mello, Samrat Revagade,
Kumar Rajeev Rastogi, Abhijit Menon-Sen, and me.