log error when handler fails

[alexanderbright]

This pull request adds writing a log message when external task fails

[codecov-commenter]

Codecov Report

Merging #58 (aaf42a1) into master (2f3bc01) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #58   +/-   ##
=======================================
  Coverage   92.32%   92.32%           
=======================================
  Files          34       34           
  Lines         352      352           
  Branches       11       11           
=======================================
  Hits          325      325           
  Misses         21       21           
  Partials        6        6           

Impacted Files	Coverage Δ
src/Camunda.Worker/Execution/HandlerInvoker.cs	100.00% <ø> (ø)

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[TechnoBerry]

Hello! Thank you for your contribution!