Add --strict flag to fail the build on missing resources

[ERussel]

Description

It would be helpful to have a --strict flag in R.swift to make the build fail when resources (e.g., missing strings, images, etc.) are not found. This is particularly useful for ensuring that all necessary resources are present in production (Release configuration).

Current Behavior

Currently, R.swift does not fail the build when resources are missing. Instead, it logs warnings and generates partial outputs.

Proposed Solution

Introduce a --strict flag that:

• Fails the build if any resource is missing.
• Logs detailed error messages about what is missing.

Use Case

• In Debug, developers might want warnings but not build failures.
• In Release, missing resources should cause a hard failure to prevent runtime issues.

[antonkhvorov]

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