bconf_io/server: use lcb_instance_config_error

For configuration errors, all code except bconf_io itself should use a
common path to handle this, as there is quite some boilerplate involved
in resetting the connection properly. Subsequent commits will travel
further down this path as well.

Change-Id: I2da18818c1c71b1c8c34ebb6d62bbbdce6be06c2
Reviewed-on: http://review.couchbase.org/29740
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>