Add error message + docs to stdin commands

[noam-sol]

cmd(workspace): add error message

Align 'workspace' cmd with 'move-node-to-workspace'.

docs(move-node-to-workspace): describe stdin usage

Similar to docs/aerospace-workspace.adoc

Test plan

1. tested with -

# current-workspace=4
echo "1\n2" | ./.debug/aerospace workspace prev # fail (now with an error message)
echo "1\n2\n4" | ./.debug/aerospace workspace prev # success

2. built docs -

[noam-sol]

On a personal note, I've been really enjoying using AeroSpace and have some other contributions that I'm planning, starting with the simpler ones to the more complicated. Probably the next one is going to be #1190 😄