Currently, ParseResult#isError only returns true if there are parse errors. If there are validation errors, it will return false. This is incorrect. Instead, isError should return true if there are either parse errors or validation errors. Furthermore, we should add new API functions (e.g. isParseError, isUnparseError, isValidationError) that can be used to determine specifically what kind of error occurred if the user only cares about one type of error (e.g. validation vs parse errors).