Actionable error message when attempting to `inspect` an unprivileged Agent as a privileged user

[kaanyalti]

Version: 8.14.0 Operating System: macos sonoma v14.5 Platform: arm64

While working on this issue comparing root and unprivileged elastic agents, I encountered an error with the inspect command when using unprivileged agent.

Steps to Reproduce:

1. Deploy ESS v8.14.0
2. Create agent policy with system integration
3. Install fleet managed agent with the unprivileged flag sudo ./elastic-agent install --unprivileged --url=<fleet url> --enrollment-token=<token>
4. Run sudo elastic-agent inspect. You should see an error similar to what's shown below

   Error: error loading agent config: error loading raw config: fail to read configuration /Library/Elastic/Agent/fleet.enc for the elastic-agent: fail to decode bytes: cipher: message authentication failed
   For help, please see our troubleshooting guide at https://www.elastic.co/guide/en/fleet/8.14/fleet-troubleshooting.html

Definition of Done

Based on discussion in https://github.com/elastic/elastic-agent/issues/4889 comments:

• [ ] When a user installs an Agent using --unprivileged, then tries to inspect this Agent with a privileged user, an error message with the suggestions in https://github.com/elastic/elastic-agent/issues/4889#issuecomment-2158635347 is shown to the user.

[kaanyalti]

cc: @ycombinator @cmacknz

[blakerouse]

Again do not use sudo the Elastic Agent is not root.

[ycombinator]

I updated the title of this issue and added a Definition of Done to make this issue about improving the error message, based on a related discussion in https://github.com/elastic/elastic-agent/issues/4889. As for documentation, I've brought that to @kilfoyle's attention via https://github.com/elastic/ingest-docs/pull/1087#issuecomment-2159456944.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)