Qiskit / qiskit-ibm-runtime / 29437376116

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Small improvements to prepare functions (#3039)

### Summary
This PR adds two small improvements to the existing functions:
- Firstly, it modifies `prepare_pec` so that when `gamma` is `1`, the
calculation of `noise_gain` does not raise a "dividing-by-zero" warning,
which currently shows up (eg in tests) when users try `prepare_pec`
passing trivial maps.
- Secondly, it adds a graceful error message to `calculate_gamma` for
the case where a ref is missing from the noise models map.
- Finally, it rewrites all the error messages for the case of missing
refs consistently.

### AI/LLM disclosure

- [x] I didn't use LLM tooling, or only used it privately.
- [ ] I used the following tool to help write this PR description:
- [ ] I used the following tool to generate or modify code:

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