download_project() fails with more than one project id

Along those lines, it would be good to have an error "can only do one at a time" in `download_project()`. Right now it's this:
```
> download_project(project_id = c("SCPCP000001", "SCPCP000020"))
Error: Project `SCPCP000001` not found.Project `SCPCP000020` not found.
```

_Originally posted by @sjspielman in https://github.com/AlexsLemonade/ScPCAr/pull/54#discussion_r3499402988_

We should either give an error or allow multiple downloads. Probably the error for now. Don't want it to be _too_ easy to kick off massive downloads.

Start at the download_project() entry point and reproduce the shown call with two project IDs. Trace how multiple IDs reach the download request, then verify that the command reports that only one project can be downloaded at a time without starting multiple downloads.

Written by the indexing model from the issue text.