Make ExternalName.SetIdentifierArgumentFn return error

### What problem are you facing?
[
`SetIdentifierArgumentFn`](https://github.com/upbound/upjet/blob/4c3f3b4/pkg/config/resource.go#L25) does not return an error since it's a simple assignment because in most of the cases we need to assign to a top level. However, more advanced functions setting value on a field path have the possibility of failure, which needs to be returned as error rather than `panic` since that function is called only in runtime - not generation time.

### How could Terrajet help solve your problem?

Change the signature to return `error`.