Report errored field name when exiting on checkFieldType

[ermik]

cayley/schema/loader.go

Lines 325 to 329 in ab2941b

	f := rt.Field(i)
	name := f.Name
	if err := checkFieldType(f.Type); err != nil {
	return err
	}

Wrap error if supporting Go 1.13

fmt.Errorf("Unsupported field '%s'; err: %w",  name, err)

Or simply inline underlying error via %v, as loader.go doesn't declare those as public error variables (e.g. ErrArrayUnsupported) to be checked against.