Update CI validation error message when verifying sha

What does this MR do and why?

This MR updates the validation message that was introduced in https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3540/diffs for fixing the security issue #417275 (closed).

The message is updated to be more accurate of the circumstances under which it appears.

This change was derived from the discussion in https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/215#note_1591850181.

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