dev-cmd/livecheck: improve error message when all formulae are autobumped

[nandahkrishna]

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When all formulae provided to livecheck are in autobump.txt, we get an uninterpretable error message, which does not help diagnose what's going on:

➜  Homebrew git:(master) brew lc prestodb
Usage: brew livecheck, lc [options] [formula|cask ...]

Check for newer versions of formulae and/or casks from upstream. If no formula
or cask argument is passed, the list of formulae and casks to check is taken
from HOMEBREW_LIVECHECK_WATCHLIST or ~/.homebrew/livecheck_watchlist.txt.

      --full-name                  Print formulae and casks with fully-qualified
                                   names.
      --tap                        Check formulae and casks within the given
                                   tap, specified as user/repo.
      --eval-all                   Evaluate all available formulae and casks,
                                   whether installed or not, to check them.
      --installed                  Check formulae and casks that are currently
                                   installed.
      --newer-only                 Show the latest version only if it's newer
                                   than the formula/cask.
      --json                       Output information in JSON format.
  -r, --resources                  Also check resources for formulae.
  -q, --quiet                      Suppress warnings, don't print a progress bar
                                   for JSON output.
      --formula, --formulae        Only check formulae.
      --cask, --casks              Only check casks.
      --extract-plist              Enable checking multiple casks with
                                   ExtractPlist strategy.
      --autobump                   Include packages that are autobumped by
                                   BrewTestBot. By default these are skipped.
  -d, --debug                      Display any debugging information.
  -v, --verbose                    Make some output more verbose.
  -h, --help                       Show this message.

Error: Invalid usage: No formulae or casks to check.

This PR attempts to improve the messaging:

➜  Homebrew git:(livecheck-error) brew lc prestodb
➜  Homebrew git:(livecheck-error) brew lc -d prestodb
/opt/homebrew/Library/Homebrew/brew.rb (Formulary::FromNameLoader): loading prestodb
/opt/homebrew/Library/Homebrew/brew.rb (Cask::CaskLoader::NullLoader): loading prestodb
==> Skipping prestodb as it is autobumped in homebrew/core.

The "skip" message only appears when using --debug but there is definitely a case to be made for printing it regardless, or a summary that all provided formulae were ignored.

[MikeMcQuaid]

Thanks @nandahkrishna!