Print usage if args not provided

[abhillman]

Resolves:

• Incorrect behavior with zero args #162
• Incorrect exit status with invalid args #163
• Invalid arguments to wattsi still give a zero exit code #77

Before:

$ ./bin/wattsi
wattsi: output directory (second argument) must be an existing empty directory
$ echo $?
0

After:

$ ./bin/wattsi
wattsi: missing required arguments
syntax:
  wattsi [--quiet] [--single-page-only] <source-file> <source-git-sha> <output-directory> <default-or-review> <mdn-spec-links/html.json> [<highlight-server-url>]
  wattsi --version
$ echo $?
1

[abhillman]

I can’t merge, but feel free or lmk if anything else needed from my end ✨