Check that all required formulae exist before processing the dependencies

Hey @andrew, the changes in this pull request deal with situations where a dependency listed in the `Brewfile` is misspelled or non-existent _(ie. no corresponding Homebrew formula can be found)_.

Before processing any of the dependencies listed in a Brewfile, brewdler will now check that the currently installed version of homebrew includes a formula for every single dependency.

This avoids confusing error messages downstream when brewdler is trying to execute the `brew list` or the `brew info` command for a dependency that doesn't have a corresponding brew formula.

Thanks, @pvdb

EDIT - BTW, the [whitespace-suppressed diff](https://github.com/andrew/brewdler/pull/8/files?w=1) is easier on the eye :smile: