Improve unset pool error

[mattnibs]

Closes #3309

[philrz]

This enhancement was in reaction to a user's experience in a community Slack thread. In their own words:

zed log isn’t working for me right now … dunno if I’m just missing something or what. I’ve tried a couple of things to troubleshoot this … (still on 1.16.0)

❯ zed init lake
lake created: file:///Users/chrismo/dev/ds6/ops/platform/aws/tf/kms/lake

❯ export ZED_LAKE=lake

❯ zed create keys
pool created: keys 2k157GhxTaVVMCOc0YGjaxgacMN

❯ aws kms list-keys --profile staging --region us-east-2 | zq -z 'over this.Keys' - | zed load -use keys -
(11/1) 792B 792B/s
2k158CfFyGJfCBFlwsXNAKFPwhf committed

❯ zed log        
empty pool and branch

They were informed of the role of zed use here and that got them up and running. But their experience did highlight that the error message could be improved to help new users bumping into this for the first time.