Add "Resource Not Found" errors in opslevel-go

[rocktavious]

Customer calling code looks like this

	service, err := client.GetService(serviceIdentifier)
	if err != nil {
		resp.Diagnostics.AddError("opslevel client error", fmt.Sprintf("Unable to read service, got error: %s", err))
		return
	}

But there are cases where service == nil and service.Id == nil which this doesn't handle which means it passes the client code error handling and they have to do this

	service, err := client.GetService(serviceIdentifier)
	if err != nil || service == nil || service.Id == nil {
		resp.Diagnostics.AddError("opslevel client error", fmt.Sprintf("Unable to read service, got error: %s", err))
		return
	}

We should fix this by adding a custom "Resource Not Found" error that we emit when service == nil || service.Id == nil but it should be done in the client.GetService function.

Then wash, rinse, repeat for all "get" functions in the opslevel-go client.