Should a missing or misspelled target generate a warning?

Should a missing or misspelled target generate a warning? Here is an example:

```r
library(targets)

tar_dir({
  tar_script(list(tar_target(real_target, 1 + 1)))
  tar_make(names = c("real_target", "nonexistent_target"))
})
# ✔ ended pipeline [53ms, 1 completed, 0 skipped]
```

Same for unquoted names, of course.

