[image_spec]: Fail when image existence check fails

[j93hahn]

Why are the changes needed?

When ImageSpec.exist() returns None (failed to verify if image exists due to Docker not running, AWS CLI issues, network problems, etc.), the previous behavior was to assume the image exists and skip building. This caused ErrImagePull errors at runtime when the image didn't actually exist in the registry.

Example error that triggered this fix:

Warning  Failed  kubelet  Failed to pull image "472386928882.dkr.ecr.us-west-2.amazonaws.com/exa-hephaestus/flytekit:LPFpPiQrpf5Xk_3oyZC58Q": 
  rpc error: code = NotFound desc = failed to pull and unpack image: not found
Warning  Failed  kubelet  Error: ErrImagePull

What changes were proposed in this pull request?

Changed the behavior in ImageSpecBuilder.should_build() when exist() returns None:

• Before: Assumed image exists, skipped building, printed blue message
• After: Raises RuntimeError with a clear message explaining how to fix the issue

This fails fast and surfaces the real problem (Docker/AWS not configured) immediately, rather than failing later at pod scheduling time with a confusing ErrImagePull error.

How was this patch tested?

Manual verification of the logic change. The fix is minimal and the behavior is straightforward.

Human review checklist

• Verify this is the desired behavior (fail fast vs. silently assume image exists)
• Check if the error message is helpful: suggests ensuring Docker/AWS CLI are configured, or using FLYTE_FORCE_PUSH_IMAGE_SPEC=1 to force rebuild

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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Link to Devin run: https://app.devin.ai/sessions/2d0f2b4119614e8db4c39e002b676ce8
Requested by: @j93hahn

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