Standardize and clarify apt mirror diagnostics in misc/build.func and misc/install.func: update warning phrasing when apt-get update fails; change per-mirror failure messages from "Mirror X: <reason>" to "Mirror X failed (<reason>)"; change success messages to "CDN set to <mirror>: tests passed"; and add informational logs like "Attempting mirror: <mirror>" when trying alternate mirrors. These changes improve visibility into mirror selection and failure reasons during package update/installation.
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