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There are way too many errors printed when running demo.

Steps to reproduce:

1. rake docker_up
2. add agent-kea, agent-kea-ha1, agent-kea-ha2

Here's a couple of errors:

agent-kea-ha2_1  | ERRO[2020-04-28 11:40:41]  promkeaexporter.go:488   cannot get stat from daemon: response result from Kea != 0: 1, text: unable to forward command to the dhcp6 service: No such file or directory. The server is likely to be offline
agent-kea-ha2_1  | isc.org/stork/agent.(*PromKeaExporter).setDaemonStats
agent-kea-ha2_1  | 	/home/thomson/devel/stork/backend/agent/promkeaexporter.go:345
agent-kea-ha2_1  | isc.org/stork/agent.(*PromKeaExporter).collectStats
agent-kea-ha2_1  | 	/home/thomson/devel/stork/backend/agent/promkeaexporter.go:486
agent-kea-ha2_1  | isc.org/stork/agent.(*PromKeaExporter).statsCollectorLoop
agent-kea-ha2_1  | 	/home/thomson/devel/stork/backend/agent/promkeaexporter.go:313
agent-kea-ha2_1  | runtime.goexit
agent-kea-ha2_1  | 	/home/thomson/devel/stork/tools/1.13.5/go/src/runtime/asm_amd64.s:1357 

gent-kea_1      | time="2020-04-28T11:40:22Z" level=error msg="ERROR: diskstats collector failed after 0.001589s: invalid line for /proc/diskstats for sdh" source="collector.go:123" 
agent-kea_1      | time="2020-04-28T11:40:22Z" level=error msg="error gathering metrics: collected metric node_hwmon_temp_celsius label:<name:\"chip\" value:\"thermal_thermal_zone0\" > label:<name:\"sensor\" value:\"temp0\" > gauge:<value:27.8 >  has help \"Hardware monitor for temperature ()\" but should have \"Hardware monitor for temperature (input)\"
agent-kea_1      | " source="<autogenerated>:1" 
agent-kea_1      | INFO[2020-04-28 11:40:22]  promkeaexporter.go:444   APP &{Type:kea AccessPoints:[{Type:control Address:127.0.0.1 Port:8000 Key:}]} 

agent-bind9_1    | * collected metric node_hwmon_temp_celsius label:<name:\"chip\" value:\"platform_coretemp_0\" > label:<name:\"sensor\" value:\"temp3\" > gauge:<value:41 >  has help \"Hardware monitor for temperature (input)\" but should have \"Hardware monitor for temperature ()\"
agent-bind9_1    | * collected metric node_hwmon_temp_celsius label:<name:\"chip\" value:\"platform_coretemp_0\" > label:<name:\"sensor\" value:\"temp4\" > gauge:<value:43 >  has help \"Hardware monitor for temperature (input)\" but should have \"Hardware monitor for temperature ()\"
agent-bind9_1    | * collected metric node_hwmon_temp_celsius label:<name:\"chip\" value:\"platform_nct6775_656\" > label:<name:\"sensor\" value:\"temp8\" > gauge:<value:0 >  has help \"Hardware monitor for temperature (input)\" but should have \"Hardware monitor for temperature ()\"
agent-bind9_1    | * collected metric node_hwmon_temp_celsius label:<name:\"chip\" value:\"platform_nct6775_656\" > label:<name:\"sensor\" value:\"temp1\" > gauge:<value:44 >  has help \"Hardware monitor for temperature (input)\" but should have \"Hardware monitor for temperature ()\"

There may be more. I think we should catch exceptions and log something more reasonable.