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!

[pditommaso]

The problem with this is that ignoreThenFail is longer than 10 characters and will not fit the Platform schema

[bentsherman]

ignoreFail ? 😆

unfortunately the shorter one is not as clear

[pditommaso]

Was thinking a couple of other implementation alternatives:

1. use ignoreThenFail but map it to IGNORE enum value and tracking the fail flag independently
2. instead of introducing new errorStrategy value, define it as an executor or nextflow scope setting

[bentsherman]

I will review the docs and then it should be good to go