Improve error handling in arc install-certificate

Summary: Fixes T9858. Reasonable typos and misunderstandings currently produce very confusing error messages.

Test Plan:

$ arc install certificate
Usage Exception: Server URI "certificate" must include a protocol and domain. It should be in the form "https://phabricator.example.com/".

• Also used a good URI.
• Also used no URI.

Reviewers: joshuaspence, chad

Reviewed By: chad

Maniphest Tasks: T9858

Differential Revision: https://secure.phabricator.com/D14577