exceptions: hint when a download is HTML, not a binary

When a download URL silently returns an HTML page (a bot-protection
challenge such as Anubis or Cloudflare, a rate-limit page, or an error
page served as `text/html`) instead of the expected binary artifact,
`ChecksumMismatchError` was reported with no clue that no real download
happened. Users were steered toward filing a stale-checksum PR.

Detect HTML in the first 512 bytes of the file and append a dedicated
hint to the existing checksum-mismatch message, including a copy-pasteable
`rm "$(brew --cache)/..."` command for retrying. The original error path
is unchanged for non-HTML downloads.

The same code path is shared by cask, formula, bottle and resource
downloads, so all of them benefit.

Discussion: https://github.com/orgs/Homebrew/discussions/6837