fix: do not print find errors

[nozaq]

Summary

Do not print errors find command produces when no lib/commands exists under plugin directories.

Fixes: #1101

Other Information

Example output from asdf help on the current master branch.

...

RESOURCES
GitHub: https://github.com/asdf-vm/asdf
Docs:   https://asdf-vm.com
find: ‘/home/_/.asdf/plugins/deno/lib/commands’: No such file or directory

PLUGIN direnv
  asdf direnv
  asdf direnv hook asdf
find: ‘/home/_/.asdf/plugins/nodejs/lib/commands’: No such file or directory
find: ‘/home/_/.asdf/plugins/python/lib/commands’: No such file or directory
find: ‘/home/_/.asdf/plugins/ruby/lib/commands’: No such file or directory
find: ‘/home/_/.asdf/plugins/terraform/lib/commands’: No such file or directory

"Late but latest"
-- Rajinikanth

After applying this fix.

...

RESOURCES
GitHub: https://github.com/asdf-vm/asdf
Docs:   https://asdf-vm.com

PLUGIN direnv
  asdf direnv
  asdf direnv hook asdf

"Late but latest"
-- Rajinikanth

[nozaq]

Just pushed a fix for shfmt check.

[jthegedus]

LGTM, thanks for catching.