add missing label when ListObjectsV2 call returns an error

[qaoru]

This fixes a panic occuring when the ListObjectsV2 call returns an error (bucket does not exist, user not authorized, ...).

INFO[0000] Listening on :9340                            source="s3_exporter.go:301"
ERRO[0014] NoSuchBucket: The specified bucket does not exist
        status code: 404, request id: 16F4DB470BC51C7C, host id:   source="s3_exporter.go:112"
panic: inconsistent label cardinality: expected 3 label values but got 2 in []string{"foobar", ""}

goroutine 39 [running]:
github.com/prometheus/client_golang/prometheus.MustNewConstMetric(...)
        /home/pheng/Git/github/s3_exporter/vendor/github.com/prometheus/client_golang/prometheus/value.go:107
main.(*Exporter).Collect(0xc000310440, 0xc000370f60?)
        /home/pheng/Git/github/s3_exporter/s3_exporter.go:113 +0x3eb
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
        /home/pheng/Git/github/s3_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:444 +0xfb
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather
        /home/pheng/Git/github/s3_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:455 +0x4c5