cmd/--cache: avoid exception on missing HEAD

[cho-m]

Also add cask to description

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---

Original T.must is not guaranteed and will lead to less readable exceptions, e.g.

❯ brew --cache --HEAD [email protected]
Error: Passed `nil` into T.must
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11581/lib/types/_types.rb:222:in `must'
/opt/homebrew/Library/Homebrew/cmd/--cache.rb:116:in `print_formula_cache'
/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.3.0/gems/sorbet-runtime-0.5.11581/lib/types/private/methods/call_validation.rb:270:in `bind_call'

Instead, this change just outputs a warning similar to missing bottle scenario. Message used is same as what is output when running brew install --HEAD on missing HEAD.

❯ brew --cache --HEAD [email protected]
Warning: No head is defined for [email protected].

[Rylan12]

Nice work!

[MikeMcQuaid]

Thanks again @cho-m!