Add logger to mergeSchemas

[pyrossh]

I need to debug errors during development and makeExecutableSchema has this nice logger parameter to log the stack traces of the errors which get swallowed by graphql.

But since the new 2.5.0 version and stitching schema's I need to pass the same logger parameter to all my schemas. Since mergeSchemas is a central place can we have the the logger parameter there also and use that to log all errors instead of repeating it always.

[freiksenet]

Sounds like a great idea, that's something we definitely want. And we definitely would welcome PRs that would implement it.

[stubailo]

I think we want to improve error handling overall, and will probably be deprecating the logger parameter. But we want to make sure we have a good replacement before we do. #887

In general, I'm also going to be dramatically improving error handling in schema stitching overall. Follow along here: #890