zfs collector reports errors on scrape if ZFS not available

[mdlayher]

Since it's enabled by default, it should be silenced if ZFS is not available on the machine.

Server is an Ubuntu 14.04 droplet on DigitalOcean.

$ ./node_exporter 
INFO[0000] Starting node_exporter (version=, branch=, revision=)  source="node_exporter.go:136"
INFO[0000] Build context (go=go1.7.4, user=, date=)      source="node_exporter.go:137"
INFO[0000] No directory specified, see --collector.textfile.directory  source="textfile.go:57"
INFO[0000] Enabled collectors:                           source="node_exporter.go:156"
INFO[0000]  - diskstats                                  source="node_exporter.go:158"
INFO[0000]  - entropy                                    source="node_exporter.go:158"
INFO[0000]  - netdev                                     source="node_exporter.go:158"
INFO[0000]  - zfs                                        source="node_exporter.go:158"
INFO[0000]  - conntrack                                  source="node_exporter.go:158"
INFO[0000]  - loadavg                                    source="node_exporter.go:158"
INFO[0000]  - edac                                       source="node_exporter.go:158"
INFO[0000]  - filesystem                                 source="node_exporter.go:158"
INFO[0000]  - meminfo                                    source="node_exporter.go:158"
INFO[0000]  - netstat                                    source="node_exporter.go:158"
INFO[0000]  - sockstat                                   source="node_exporter.go:158"
INFO[0000]  - stat                                       source="node_exporter.go:158"
INFO[0000]  - textfile                                   source="node_exporter.go:158"
INFO[0000]  - vmstat                                     source="node_exporter.go:158"
INFO[0000]  - filefd                                     source="node_exporter.go:158"
INFO[0000]  - mdadm                                      source="node_exporter.go:158"
INFO[0000]  - time                                       source="node_exporter.go:158"
INFO[0000]  - uname                                      source="node_exporter.go:158"
INFO[0000]  - hwmon                                      source="node_exporter.go:158"
INFO[0000] Listening on :9100                            source="node_exporter.go:176"
ERRO[0003] ERROR: zfs collector failed after 0.003063s: ZFS / ZFS statistics are not available  source="node_exporter.go:92"

[SuperQ]

Let's reduce this to a Debug error.