Improve user-facing error messages, fix broken upgrade command on linux

Exercism Command-Line Interface (CLI)

Exercism takes place in two places: the discussions happen on the website, and you work on exercises locally. The CLI bridges the gap, allowing you to fetch exercises and submit solutions to the site.

This is a stand-alone binary, which means that you don't need to install any particular language or environment in order to use it.

To install, download the archive that matches your operating system and architecture, unpack the archive, and put the binary somewhere on your path.

You will need to configure the CLI with your Exercism API Key before submitting.

For more detailed instructions, see the CLI page on Exercism.

Recent Changes

• If you try to submit a directory to exercism, the error message now makes sense.
• The output from the debug command is improved for users who have not configured their API key.
• Upgrade command was broken on Linux, this is now fixed.
• The list of commands in the usage statement has been alphabetized, which makes it a bit easier to read/navigate.
• Improve the "fetch" hint in the list command using the first problem in the language listed as the example.

There were also a number of improvements to the contributing documentation.