suprise check for `pipeline` vs `pipelines`

[cyrush]

snippet user actions intending to run a sampling pipeline and save reduced result using a blueprint extract:

-
  action: "add_extracts"
  extracts: 
    e1:
      type: "relay"
      pipelines: "pl1"
      params: 
        ....

In this pipelines is incorrect, the user intended to provide pipeline * since pipeline is missing -- he the full dataset is saved instead of the sampling result

we should let folks know that pipelines is a surprise and subtlety wrong.