Error when rapidly cycling through previews

[olidacombe]

E.g.

E5108: Error executing lua .../site/pack/packer/start/plenary.nvim/lua/plenary/job.lua:405: Failed to spawn process: {
  _additional_on_exit_callbacks = {},
  _shutdown_check = <userdata 1>,
  _stderr_results = {},
  _stdout_results = {},
  args = { "-f", "dragon" },
  command = "cowsay",
  enable_handlers = true,
  enable_recording = true,
  interactive = true,
  pid = "EMFILE: too many open files",
  stderr = <userdata 2>,
  stdin = <userdata 3>,
  stdout = <userdata 4>,
  user_data = {},
  writer = { "ok" },
...

too many open files suggests we might need a finite pool of Jobs, and we render "loading..." in the preview buffer before updating with the rendered result when ready. Or hopefully there's a simpler solution. I've only scratched the surface of :help telescope.previewers and I know there are at least some cache keys to take care of.