Register port file after connecting to port

It's currently possible to add a port file to `s:repl_portfiles` that doesn't have a `connection` key (ie. when trying to eval something in a project that still has an old `.nrepl-port` in it).

Unfortunately, there's a couple places that expect that `connection` key to be there, leading to a `Vim(call):E716: Key not present in Dictionary: connect)` error on any subsequent eval, even if the REPL does get started.

This pull fixes this behaviour by only adding an entry into `s:repl_portfiles` after successfully connecting to the REPL.

An alternative fix would have been to guard each of those dictionary accesses with `has_key(..., 'connection')` but then the port file would get registered once and then would never get tried again, even after the REPL has started. The solution submitted above makes more sense to me.