Add a test for the err package

[negz]

And various other fixes.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@cfe9594). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #2   +/-   ##
=========================================
  Coverage          ?   69.53%           
=========================================
  Files             ?       13           
  Lines             ?      384           
  Branches          ?        0           
=========================================
  Hits              ?      267           
  Misses            ?      116           
  Partials          ?        1

Impacted Files	Coverage Δ
memory/fifo.go	85.71% <ø> (ø)
manager/manager.go	100% <ø> (ø)
queue.go	0% <ø> (ø)
metrics/queue.go	84.21% <ø> (ø)
logging/manager.go	91.66% <ø> (ø)

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