Error messages have non-standard capitalization and quote usage

For example, sometimes we capitalize the types of GraphQL schema elements (e.g. Object type) and sometimes we do not.

Sometimes we quote the type name, field name, etc, and sometimes we do not.

Within https://github.com/graphql/graphql-js/pull/3044 @leebyron removed many quotes => unclear if that's intentional, but seems to look nicer. The rebased version does not make this change, preferring to pull any quotes change out into a separate PR, and hopefully be consistent across the entire code base.

See also: https://github.com/graphql/graphql-js/pull/4195#pullrequestreview-2314880635