Prevent FATAL error when non-existing prepared statement is given.

Prevent FATAL error when non-existing prepared statement is given.

Previously Bind() raised FATAL error if non-existing prepared statement was given.
This is different from PostgreSQL's behavior.

\bind_named stmt2 'baz' \g
ERROR:  prepared statement "stmt2" does not exist

This commit let Bind() check the existence of prepared statement and
if it does not exist, send an ERROR message to frontend. Note that no
log is left in the log file. This is not preferred action but currently
it's not possible to leave log.

Backpatch-through: v4.2