add onerr callback for leader elector

[tg123]

#1390

[codecov-commenter]

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@@            Coverage Diff            @@
##             master    #1399   +/-   ##
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  Coverage          ?   70.31%           
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  Files             ?       90           
  Lines             ?     2712           
  Branches          ?        0           
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  Hits              ?     1907           
  Misses            ?      805           
  Partials          ?        0           

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[brendandburns]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, tg123

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