Improve coercion error messages
Improve coercion error messages

* Remove redundant falsy check for runtimeType

* Replace silent conversion to null with an invariant and a useful error message

* Add an error message for unexpected return value from resolveType

* Make sure null and undefined mean could not determine type while false or 0 indicates wrong return type

jjergus/graphql-js,jjergus/graphql-js,tgriesser/graphql-js,gabelevi/graphql-js,josephsavona/graphql-js,sogko/graphql-js,graphql/graphql-js,enaqx/graphql-js,sogko/graphql-js,graphql/graphql-js,enaqx/graphql-js,baer/graphql-js,josephsavona/graphql-js,gabelevi/graphql-js,sogko/graphql-js,graphql/graphql-js,enaqx/graphql-js,tgriesser/graphql-js,baer/graphql-js,josephsavona/graphql-js