User-friendly error messages for invalid options/arguments; fixes #5997

- Show a more user-friendly error message when dealing with an option whose (mandatory) argument is missing.

- Likewise, we show a more user-friendly error message when a given option is ambiguous. Examples:
    - Given `-f`, the parser successfully expands to `--fontdir`.

    - Given `-c`, we fail to expand because the parser can’t tell if the user means `--caskroom` or rather `--colorpickerdir`. This exception now results in a nicer error message.

- Some commands now result in a consistent error message when a required cask name is missing.
This affects all stable commands which require one or more cask names as arguments, i. e. `cat`, `create`, `edit`, `fetch`, `info`, `install`, `uninstall`, and `zap`.

- Up to now, the commands `cat`, `create`, `edit`, and `info` used to treat any unknown option as a cask name. This commit changes that behaviour to make it consistent to the rest of the commands (like `fetch`, `install`, `uninstall`, and `zap`), who have silently discarded any unknown option in the past, and continue to do so.
Add tests for #5997
Rewrite tests to make them work with Ruby 1.8
Merge pull request #6491 from claui/issue-5997

User-friendly error messages for invalid options/arguments; fixes #5997