[receiver/podman] Invalid stats error handler

[rogercoll]

I was working on a new feature for the podman receiver and I noticed that if the containers stats API endpoint returns an error, the receiver panics:

2022-04-21T20:48:51.763+0200	error	podmanreceiver@v0.49.0/receiver.go:80	error fetching stats	{"kind": "receiver", "name": "podman_stats", "error": "json: cannot unmarshal object into Go struct field containerStatsReport.Error of type string"}
github.com/open-telemetry/opentelemetry-collector-contrib/receiver/podmanreceiver.(*receiver).scrape
	github.com/open-telemetry/opentelemetry-collector-contrib/receiver/podmanreceiver@v0.49.0/receiver.go:80
go.opentelemetry.io/collector/receiver/scraperhelper.ScrapeFunc.Scrape
	go.opentelemetry.io/collector@v0.49.0/receiver/scraperhelper/scraper.go:32
go.opentelemetry.io/collector/receiver/scraperhelper.(*controller).scrapeMetricsAndReport
	go.opentelemetry.io/collector@v0.49.0/receiver/scraperhelper/scrapercontroller.go:195
go.opentelemetry.io/collector/receiver/scraperhelper.(*controller).startScraping.func1
	go.opentelemetry.io/collector@v0.49.0/receiver/scraperhelper/scrapercontroller.go:173
2022-04-21T20:48:51.763+0200	error	scraperhelper/scrapercontroller.go:198	Error scraping metrics	{"kind": "receiver", "name": "podman_stats", "error": "json: cannot unmarshal object into Go struct field containerStatsReport.Error of type string", "scraper": "podman_stats"}
go.opentelemetry.io/collector/receiver/scraperhelper.(*controller).scrapeMetricsAndReport
	go.opentelemetry.io/collector@v0.49.0/receiver/scraperhelper/scrapercontroller.go:198
go.opentelemetry.io/collector/receiver/scraperhelper.(*controller).startScraping.func1
	go.opentelemetry.io/collector@v0.49.0/receiver/scraperhelper/scrapercontroller.go:173

The error is because the actual structure used to unmarshal the api response:

type containerStatsReport struct {
	Error string
	Stats []containerStats
}

As the Podman API documentation describes, the Error is a structure with three attributes:
https://docs.podman.io/en/latest/_static/api.html#operation/ContainersStatsAllLibpod