This patch adds a new StatementErrored method to the stmtcache.
This routine MUST be called by users of the cache whenever the
execution of a statement results in an error. This will allow
the cache to make an intelligent decision about whether or not
the statement needs to be purged from the cache.
PreparedStatementDescription was too long. It also no longer entirely
represents its purpose now that it is also intended for use with
described statements.
