Improve error message when no script name is given

If no script name is given, cardboard will end up with strange error as it will try to execute the parent directory:

```
../.bundle/ruby/2.1.0/gems/cardboard-1.0.4/bin/cardboard:13:in `exec': Permission denied - ../.bundle/ruby/2.1.0/gems/cardboard-1.0.4/libexec (Errno::EACCES)
	from ../.bundle/ruby/2.1.0/gems/cardboard-1.0.4/bin/cardboard:13:in `<top (required)>'
	from ../.bundle/ruby/2.1.0/bin/cardboard:23:in `load'
	from ../.bundle/ruby/2.1.0/bin/cardboard:23:in `<main>'
```

Proposed PR will print a simple usage with all available script names if no script name is given.
Example below.

```
cardboard: No script given.
usage: cardboard <script>

Available scripts are:
   bootstrap
   cibuild
   lint
   specs
   syntax
```