improve JSONSchemaType errors
improve JSONSchemaType errors

typescript seems to report errors with the last member of a valid union
first, so this rearranges the type so the simple union types come first,
and the type that represents the meaty types comes last. This should
help make errors like the one in #1521 more clear.

epoberezkin/ajv,epoberezkin/ajv,epoberezkin/ajv