[schema] Validate that type name is a string

[rexxars]

If you have an error in your schema where a type-field is a non-string, the schema validation will fail when trying to figure out the closest match to it it with a levenshtein distance comparator.

This adds an additional step ensuring that the type is a string.

[thomax]

LGTM. Though I'm wondering if it would be even more helpful if the current typeName was included in the error message?

[rexxars]

typeName could be a big object of some kind, so I'm not sure how that would work. Potentially serialize it to JSON and cut out a part of it?

[rexxars]

The schema validator does include a parent path, however, so it shouldn't be too hard to track down.