…reesInvesting/r2dii.plot into 205-add-main-arg-trajectory

* Refactor and new helper `abort_if_invalid_length()`
* Use backtics to format variables (reserve quotes for values).
* The error message now prints correctly -- starting with a bullet.
* `any()` is missleading, as `main_line` should have length-1 (?)
* New helper `abort_if_invalid_length()`
* Classify `abort_if_invalid_length()`
* Rename for consistency
* Reword argument description for consistency
* Rename for consistency
* Side-effect functions should return invisible()
* Rename for consistency
* Polish docs