Improve error handling in the Pipeline for multiple accessor patterns

[tennlee]

When creating a pipeline with two accessors, they must be declared as a tuple. It is easy to accidentally not do this and just put then as though they were pipeline steps.

c.f. Pipeline(a, b) vs Pipeline((a, b))

In this case, the error message is not particularly informative. The system should recognise when two accessors are chained as steps, and report a more specific error message to alert the user to this potential situation.