feat: Don't panic when api impl is nil

[placer14]

(fixes #6632)

curl -X POST -i -H "Content-Type: application/json" --data '{ "jsonrpc": "2.0", "id":1, "method": "Filecoin.MpoolGetNonce", "params": [ "t1mps3lifbqregwqnfbcyjlllectulzgchbhsvomy" ] }' 'http://127.0.0.1:2346/rpc/v0'
HTTP/1.1 200 OK
Date: Fri, 23 Jul 2021 23:17:42 GMT
Content-Length: 77
Content-Type: text/plain; charset=utf-8

{"jsonrpc":"2.0","id":1,"error":{"code":1,"message":"method not supported"}}

[placer14]

This removes the panicking, BUT it's unclear how to manage the HTTP status code and the error code returned in the payload. This also checks in the artifacts from make gen. LMK if another process handles this and should be removed from this PR.

[magik6k]

Http 200 code is correct even for method-not-found jsonrpc errors. We could use https://github.com/filecoin-project/go-jsonrpc/pull/36/files to return error codes, but I don't think that's necessary

This also checks in the artifacts from make gen

That's good, those are meant to be checked in

[magik6k]

Looks like a nice improvement