Report errored field name when exiting on checkFieldType

[ermik]

https://github.com/cayleygraph/cayley/blob/ab2941b35ef5088e26e1350e504fd55ba9630358/schema/loader.go#L325-L329

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.