Enable partial errors

[spawnia]

Is your feature request related to a problem? Please describe.

I ended up having a problem similar to #407 in our project.

We have a resolver that may return partially correct data, as well as some warnings. Right now, Lighthouse can either return data or errors, never both.

Describe the solution you'd like

I would like to be able to add errors to the result without stopping execution. We would have to add a mechanism that does not rely on Exceptions but rather allows to put errors in some kind of buffer that is collected before ending resolution.

[joskfg]

That happens to me too. I'm calling an external graphql server from a graphql server, so the call could return some errors that I'd like to put in the output. #865 would help too.

[spawnia]

Resolved through #1420