Report errored field name when exiting on checkFieldType

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

[Wrap](https://blog.golang.org/go1.13-errors) error if supporting Go 1.13
```go
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.

Start in schema/loader.go around lines 325-329 and trace where checkFieldType returns its error. Update the resulting error so it identifies the unsupported field while preserving the underlying error, then verify that loader failures report the field name.

Written by the indexing model from the issue text.