Similar to #2486. This will be used for SDK-3181.
```
- WARN: ⚠️ Returning fake signature verification failure for 'GET /v1/subscribers/$RCAnonymousID:fc0508bea4a74a41bfc9a69db30d925f'
- ERROR: 😿‼️ Request failed signature verification. Request to /v1/subscribers/$RCAnonymousID%3Afc0508bea4a74a41bfc9a69db30d925f failed verification
```
Scenario:
- Response 1: 200 / `VerificationResult.failed`
- Response 2: 304 / `VerificationResult.verified`
Before this change, that was resulting in `VerificationResult.verified`,
even though it was using the unverified response body.
This fixes that so that we don't use the ETag for a response that failed
validation, even in `ResponseVerificationMode.informational`.