Add line numbers to error messaging

`CheckpointError`s raised in [checkpoint.py](https://github.com/Parcels-code/virtualship/blob/main/src/virtualship/models/checkpoint.py) tell the user that there's an issue concerning waypoint x. However, if the expedition is long it can be cumbersome to locate this waypoint in the YAML. Adding a line number associated with the problematic waypoint to the error message would help with this.