Show appropriate error messages on trying skipping/unmarking commands with no fake cursors

When no fake cursors exist, I run `mc/unmark-next-like-this` and get an error message:
```
Wrong type argument: overlayp, nil
```
This is confusing.
This pull request refines error messages raised from `mc/mark-next-like-this`, `mc/mark-previous-like-this` and `mc/mark-more-like-this` when no fake cursors exist.

Suitable tests should be added, but I don't know how ecukes handles errors...