Add ignoreThenFail error strategy

[bentsherman]

Close #4446

Test example:

process foo {
  errorStrategy 'ignoreThenFail'

  input:
  val id

  script:
  """
  test $id -ne 4
  """
}

workflow {
  foo( Channel.of(1..10) )
}

This pipeline will ignore the failed task but then return a non-zero exit code.

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[pditommaso]

Tests are needed

[adamrtalbot]

Hey everyone, can I restart this one a bit? It would be a very useful feature!