make error message more useful when passing a 0-length grouping variable

[kendonB]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Proposal

When I pass a grouping variable of length 0, the current error message is There are only 0 elements.

I propose to change this to There are only 0 elements in grouping variable <object_name>.

library(drake)
my_plan <- drake_plan(
  i = numeric(),
  mytarget = target({
    1
  }, dynamic = map(i))
)
make(my_plan)
#> ▶ target i
#> ▶ dynamic mytarget
#> > subtarget mytarget_
#> Error: Must index existing elements.
#> x Can't subset position 1.
#> ℹ There are only 0 elements.

^{Created on 2020-03-11 by the reprex package (v0.3.0)}