Throw error if mock response file not found

[dlarocque]

If we call getMockResponse with a filename that doesn't exist, we'll get some confusing error like not being able to read a property from undefined. It'd be better if we just threw an error if the mock response file doesn't exist.

[changeset-bot]

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Latest commit: c68a16d

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (01f36ea) and merge commit (8890a9b).

Test Logs

• Base (01f36ea): https://github.com/firebase/firebase-js-sdk/actions/runs/13021795480
• Merge (8890a9b): https://github.com/firebase/firebase-js-sdk/actions/runs/13038403458

1. https://storage.googleapis.com/firebase-sdk-metric-reports/pVy1YJxh7a.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (01f36ea) and merge commit (8890a9b).

Test Logs

• Base (01f36ea): https://github.com/firebase/firebase-js-sdk/actions/runs/13021795480
• Merge (8890a9b): https://github.com/firebase/firebase-js-sdk/actions/runs/13038403458

1. https://storage.googleapis.com/firebase-sdk-metric-reports/yZ0RvLoARl.html