Add Error History function and config var.

Add Error History function and config var.

<error-history>, by default unbound, shows a list of the recent error
messages displayed by Mutt via mutt_message() or mutt_error().

$error_history sets the size of the history ring.

For now, I've decided to include mutt_message() messages too.  If this
is too chatty, we can restrict it to mutt_error() easily in the
future.