Feature: Pipelining and steps flow control (try & catch)

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YAML example:

pipelines:
    :build:
        - task: :build --php=8.0
        - task: :migrations
          error: :notify "Migrations failed"
        - task: :run
          rescue: :when-run-fails
        - block:
              - task: :something
          rescue: :notify 
        - task: :something

Shell example:

rkd \
     :build --php=8.0 \
     {@failure :notify "Migrations failed"} :migrations {@failure} \ 
     {@rescue :something} :run {@rescue} \ 
     :something

Python example:

# todo

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Alternatively:

• Allow to prefix steps with "on_error" / "catch" , and "always" / "finally" to execute steps depending on if there was an error or not

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Mostly done by @dkwebbie, now needs to be adapted to YAML syntax.