formula_installer: handle attempts to load invalid casks

[mistydemeo]

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Certain casks in the main cask repository may be invalid, for whatever reason. Without handling this exception, the user will get a cask-related exception while the formula of the same name is installed, and their installation will abort half-completed.

For example, the fish cask is currently broken and attempting to brew cask info fish will return an error. This also leads to attempts to install the fish formula to fail like this:

==> Upgrading fish
==> Downloading https://homebrew.bintray.com/bottles/fish-3.0.1.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Pouring fish-3.0.1.mojave.bottle.tar.gz
Error: Cask 'fish' definition is invalid: invalid 'depends_on macos' value: ":snow_leopard"

Fixes #5709.

[MikeMcQuaid]

Nice catch, thanks @mistydemeo!