Up to now, we haven't been accounting for `#url` symbol arguments in
`livecheck` blocks that don't reference a checkable URL. This can
either be an invalid symbol (e.g., using the `:stable` formula symbol
in a cask) or a valid symbol where the referenced URL doesn't exist
(e.g., using `:head` when there's no `head` URL). [Almost all of the
valid symbols are required URLs but `head` is optional.]
Over the years, we've had a handful of slips where we've used `:url`
in formulae (when it's only valid in casks) and `:stable` in casks
(when it's only valid in formulae). In this scenario,
`livecheck_url_string` is `nil`, so livecheck falls back to
`#checkable_urls`. In this scenario, `stable` and `url` are the first
checkable URLs for formulae and casks (respectively), so the checks
ended up working as expected merely by chance. This isn't obvious in
the output and even the debug output looks normal. It only becomes
apparent that livecheck isn't working as expected if it iterates
through more than one checkable URL before reaching one that works
(not the case in those instances).
With that in mind, this adds an error when a `#url` symbol is used
but it doesn't correspond to a checkable URL. This will account for
both of the mentioned scenarios (invalid symbols and valid ones
referencing a non-existent URL) and prevent livecheck from quietly
proceeding in an unexpected manner.
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