Leaving out name field in resolver has confusing error message

- You get a very unhelpful error if your resolver file does not have a `name:` field

_Originally posted by @codygman in https://github.com/input-output-hk/haskell.nix/issues/1157#issuecomment-873181947_

Maybe we should make it optional?

The issue does not name a file, test, or entry point. Start by locating the resolver-file parsing path and reproduce the error with the name field omitted; clarify whether the intended outcome is a more helpful error or making name optional, then add coverage for the chosen behavior.

Written by the indexing model from the issue text.