[sensors] allow sensor readings when there are warnings

[anxdpanic]

New to Go, I'm not entirely sure the nil checks are required.

Before:

/app # ./glance sensors:print
Failed to retrieve list of sensors: Number of warnings: 1
 - read /sys/class/hwmon/hwmon4/temp1_input: no data available

After:

/app # ./glance sensors:print
Errors encountered while retrieving list of sensors:
 Number of warnings: 1
 - read /sys/class/hwmon/hwmon4/temp1_input: no data available
Sensors found:
 acpitz: 27.8°C
 acpitz: 29.8°C
 pch_skylake: 36.5°C
 jc42: 27.6°C
 coretemp_package_id_0: 59.0°C
 coretemp_core_0: 56.0°C
 coretemp_core_1: 59.0°C
 coretemp_core_2: 59.0°C
 coretemp_core_3: 56.0°C