Invalid manim commands should not raise error

[niekvandepas]

Apologies if this issue already exists; I could not find it.

Description of proposed feature

Current behavior is such that running an invalid manim subcommand, such as manim help, raises a Python error (FileNotFoundError: /private/tmp/project/help not found).

A better user experience would be if attempting to run an invalid command would display a user-friendly error along the lines of invalid command: ${command}.

I'm not too familiar with Python nor this codebase hence the lack of a PR. 😇

Additional comments

It may be interesting to include checking for typos (try running git iniy).