Pipeline generator: Add missing Dockerfile dependency validation (Phase 2 of #967)

[coderabbitai]

Description

Issue #967 identified that both sandbox.py and the Tekton pipeline generator (konflux_generate_component_build_pipelines.py) need better error reporting for missing Dockerfile dependencies. PR #3017 addressed sandbox.py (Phase 1), but the pipeline generator still needs similar validation.

Enhancement Request

Apply the same validation logic to konflux_generate_component_build_pipelines.py to ensure consistent error reporting across all tooling. The pipeline generator should:

• Validate that all files referenced in Dockerfiles exist before processing
• Provide clear, actionable error messages pointing to missing files
• Avoid generic FileNotFoundError stack traces

Context

This is Phase 2 of the plan outlined in issue #967. Phase 1 (sandbox.py) was completed in PR #3017.

References

• Phase 1 implementation: ISSUE #967: fix(sandbox): improve error reporting for missing Dockerfile dependencies #3017
• Discussion: ISSUE #967: fix(sandbox): improve error reporting for missing Dockerfile dependencies #3017 (comment)
• Original issue: need better reporting for files mentioned in Dockerfile but missing on disk #967

Requested by: @jiridanek