Evals: cryptic error when spans not yet propagated in CloudWatch

[notgitika]

Description

When running evals, if the spans have not yet propagated in CloudWatch, the CLI surfaces a raw API error that is cryptic and unhelpful. It should instead display a user-friendly message explaining the delay.

Steps to Reproduce

1. Run an agent invocation
2. Immediately run evals against that invocation (within ~1-2 minutes)
3. Observe the error when spans are not yet available

Expected Behavior

The CLI should display a clear, user-friendly message such as:

"Spans not found — it takes 2-3 minutes to propagate in CloudWatch. Please try again shortly."

Actual Behavior

A cryptic, raw API error is shown that does not help the user understand the issue or what to do next.

CLI Version

N/A

Operating System

macOS

[notgitika]

The cryptic missing-log-group API error is gone. Current behavior says:

No session spans found for agent "..." ... Has the agent been invoked?

PR #1404 resolved the raw error path.