Improve error when valid CSV selectors not found

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The following discussion from !110031 (merged) should be addressed:

• @cablett started a discussion: (+1 comment)

  I know this is based on existing code but it seems this error isn't very useful. It'd be nice to add an action item (e.g. "Separator is invalid - please ensure ,, ;, or tab.")

  However, this is definitely a question for the team (Importer:CSV Issues ?) whose PM is responsible for this, so no action needed just now 👍