Enhancements to the Early Return sample

[steveandroulakis]

What was changed

The Early Return sample in core/src/main/java/io/temporal/samples/earlyreturn

• Better error handling in the client (to log "io.grpc.StatusRuntimeException: PERMISSION_DENIED: ExecuteMultiOperation API is disabled on this namespace.")
• Code comments in the workflow
• Added a link to the sample in the main README

Why?

Less failure-prone and easier to understand Early Return sample.

Checklist

1. Closes

n/a

2. How was this tested:

Existing tests still apply

3. Any docs updates needed?

No