document --verbose --fail pattern better

Ever since switching otel-cli to default to silent failure, it is tricky even for me to remember I have to send the --verbose --fail flags to get it to output the error and fail as expected. This should at least be clearer in the README.

Start in the repository README and locate the usage guidance for --verbose and --fail. Explain that otel-cli defaults to silent failure and document that both flags are needed to print errors and fail as expected; done when the README makes this behavior clear.

Written by the indexing model from the issue text.