Add an option to pass a request to the next server on Lua error

tnt_next_upstream option supports only few default strategies to pass a request to the next server. In some cases one may need to do it on Lua errors.

Possible solutions are:

1. Return HTTP 500 on Lua errors and use default mask http_500.
2. Add a special mask for this case (e.g. lua_error).

Almost done but before push I wish to test this feature with balancer_by_lua - from ngx_lua.

Implementation details: -- nginx.conf { tnt_next_upstream tnt_error [masks..]; }

@toogle Sytax/Option is ok for you. And also I'm interested, did you use balancer_by_lua?

No, we didn't. Anyway, it would be great to have such functionality directly in upstream module instead of relying on ngx_lua.

We also need box.error/error support in protocol, to correct balancing stream in upstream to avoid loss data by tarantool errors.